Protection Against Email Bombs with Microsoft Defender for Office 365
In today's digital age, email remains a critical communication tool for businesses and individuals. However, with the increasing sophistication of cyberattacks, email security has become more important than ever. One such threat that has been growing is the email bombing, a form of net abuse that sends large volumes of email to an address to overflow the mailbox, overwhelm the server, or distract attention from important email messages indicating a security breach. Email bomb - Wikipedia Understanding Email Bombing Email bombing, typically involves subscribing victims to a large number of legitimate newsletter and subscription services. Each subscription service sends email notifications, which in aggregate create a large stream of emails into the victim’s inbox, making email triage for legitimate emails very difficult. This form of attack is essentially a denial-of-service (DDOS) on the victim's email triaging attention budget. Hybrid Attacks More recently, email subscription bombs have been coupled with simultaneous lures on Microsoft Teams, Zoom, or via phone calls. Attackers impersonate IT support and offer to help solve the email problem caused by the spike of unwanted emails, ultimately compromising the victim's system or installing malware on their system. This type of attack is brilliant because it creates a sense of urgency and legitimacy, making victims more likely to accept remote assistance and inadvertently allow malware planting or data theft. Read about the use of mail bombs where threat actors misused Quick Assist in social engineering attacks leading to ransomware | Microsoft Security Blog. Incidence and Purpose of Email Bombing Email bombing attacks have been around for many years but can have significant impacts on targeted individuals, such as enterprise executives, HR or finance representatives. These attacks are often used as precursors to more serious security incidents, including malware planting, ransomware, and data exfiltration. They can also mute important security alerts, making it easier for attackers to carry out fraudulent activities without detection. New Detection technology for Mail Bombing attacks To address these types of attacks Microsoft Defender has now released a comprehensive solution involving a durable block to limit the influx of emails, the majority of which are often spam. By intelligently tracking message volumes across different sources and time intervals, this new detection leverages historical patterns of the sender and signals related to spam content. It prevents mail bombs from being dropped into the user’s inbox and the messages are rather sent to the Junk folder (of Outlook). Note: Safe sender lists in Outlook continue to be honored, so emails from trustworthy sources are not unexpectedly moved to the Junk folder (in order to prevent false positives). Since the initial rollout that started in early May, we’ve seen a tremendous impact in blocking mail bombing attacks out of our customers’ inboxes: How to leverage new “Mail bombing” detection technology in SOC experiences 1. Investigation and hunting: SOC analysts can now view the new Detection technology as Mail bombing within the following surfaces: Threat Explorer, Email entity page and Advanced Hunting empowering them to investigate, filter and hunt for threats related to mail bombing. 2. Custom detection rule: To analyze the frequency and volume of attacks from mail bombing vector, or to have automated alerts configured to notify SOC user whenever there is a mail bombing attack, SOC analysts can utilize the custom detection rules in Advanced hunting by writing a KQL query using data in DetectionMethods column of EmailEvents table. Here’s a sample query to get you started: EmailEvents | where Timestamp > ago(1d) | where DetectionMethods contains "Mail bombing" | project Timestamp, NetworkMessageId, SenderFromAddress, Subject, ReportId The SOC experiences are rolled out worldwide to all customers. Conclusion Email bombs represent an incidental threat in the world of cybersecurity. With the new detection technology for Mail Bombing, Microsoft Defender for Office 365 protects users from these attacks and empowers Security Operations Center Analysts to ensure to gain visibility into such attacks and take quick actions to keep organizations safe! Note: The Mail bombing protection is available by default in Exchange Online Protection and Microsoft Defender for Office 365 plans. This blog post is associated with Message Center post MC1096885. Also read Part 2 of our blog series to learn more about protection against multi-modal attacks involving mail bombing and correlation of Microsoft Teams activity in Defender. Learn: Detection technology details table What's on the Email entity page Filterable properties in the All email view in Threat ExplorerMicrosoft Defender for Office 365: Fine-Tuning
In incident response, most business email compromise doesn’t start with “sophisticated zero-day malware.” It starts with configuration gaps: forwarding mail outside the tenant, users clicking through Safe Links warnings, impersonation policies left at day-one defaults, or post-delivery cleanup still relying on a human analyst at 2:00 AM. Those gaps are what attackers actually exploit. This blog covers our top recommendations for fine-tuning Microsoft Defender for Office 365 configuration from hundreds of deployments and recovery engagements: Core fine-tuning actions every email or security admin should land right now Data-driven bulk mail tuning (BCL and Bulk Mail Insights) Impersonation and anti-phishing policy hygiene for executive protection Automate post-delivery cleanup by enabling Automated Remediation Each section includes a short video and practical guidance you can apply immediately in Microsoft Defender for Office 365. These recommendations align with Microsoft’s “secure by default” direction: applying the Standard and Strict preset security policies to users, using Configuration analyzer to catch configuration drift, and enforcing least-privilege release of high-risk mail. When possible, enable the Preset security policies to give you Microsoft’s recommended settings for Safe Links, Safe Attachments, Anti-Phishing, and Anti-Spam. If you use custom policies (or if you exclude users from the Presets) then use Configuration analyzer regularly to compare custom policies to the Standard/Strict baselines, since those get updated as Microsoft updates the Preset policies. Core Fine-Tuning Checklist for Defender for Office 365 This section highlights six controls we recommend implementing broadly. These are “day one hardening” items we repeatedly validate with customers. Block automatic external forwarding by default Attackers often create hidden inbox rules that quietly forward mail (invoices, purchase orders, wire info) to an external account they control. Use outbound spam policies to block automatic external forwarding for the entire org and then create tightly scoped exceptions only for the handful of mailboxes that legitimately need it. This prevents data leakage and payment fraud scenarios where mail auto-forwards out of your tenant without anyone noticing. Although this setting is on by default (“System Controlled” means that external forwarding is disabled), we’ve found many tenants where this was disabled because the admin didn’t know how to create a custom policy for authorized forwarders. The trick is to order custom outbound policies to run as a higher priority than the default outbound policy which should be set to block auto-forwarded emails. It is a good idea to regularly review the auto forwarded message report (located in the Exchange Admin Center). Use Enhanced Filtering for Connectors (“skip listing”) when necessary If you’re routing inbound mail through a third-party Secure Email Gateway or an on-prem hop before Microsoft 365, Defender will see that intermediary as the source IP instead of the original sending IP, which degrades anti-spoofing effectiveness.Enhanced Filtering for Connectors — also called skip listing — lets Microsoft 365 look past that last hop and evaluate the real sending IP and headers, so SPF / DKIM / DMARC and anti-spam logic work correctly. This setting does not support centralized mail routing (unless the routing is linear; see the Enhanced Filtering for Connectors learn article), so make sure you are not using that before enabling Enhanced Filtering. Centralized routing is sometimes used by organizations running a hybrid Exchange deployment, connecting Exchange Online with an on-premises Exchange Server organization. Important: Do this instead of blanket SCL -1 transport rules that “bypass spam filtering for anything coming from our gateway.” Over-bypassing means phishing that slipped through the third-party filter can sail straight to user inboxes, which Microsoft specifically warns against. Turn on Safe Attachments protection beyond email (SharePoint, OneDrive, Teams) In the Safe Attachments “Global settings,” make sure Defender for Office 365 is set to protect files in SharePoint, OneDrive, and Microsoft Teams. When enabled, if a file is identified as malicious, Defender automatically locks the file in-place so users can’t open it in Teams or OneDrive. This gives you malware detonation and containment in collaboration channels, not just email. This step closes a gap we still see a lot: customers protect mail attachments well, but shared files and Teams chats are wide open. In the 1st part of this blog series, Microsoft MVP Purav Desai describes (here) how to prevent users from downloading malicious files by running a SharePoint PowerShell cmdlet: Set-SPOTenant -DisallowInfectedFileDownload $true Don’t let users click through Safe Links warnings Safe Links rewrites and time-of-click scans URLs in mail, Office apps, and Teams. In the Safe Links policy, clear “Let users click through to the original URL.” That prevents the classic “I know it says it’s malicious, but I really need to see it…” moment. Users get blocked instead of “warned but allowed.” This setting is also enforced in Microsoft’s Standard AND Strict preset security policies where click-through is explicitly disabled. Go beyond the default Common Attachment filter The anti-malware policy’s Common Attachment filter blocks known dangerous file extensions (executable content, scriptable content, etc.). Microsoft ships a default list (historically 50+ high-risk extensions), and you can customize it to block additional file types common in malware delivery, like HTML droppers or password-protected archives. Messages with those file types are treated as malware and quarantined. Do this centrally rather than relying on users to “spot a suspicious attachment.” Automation beats user judgment here. Use custom quarantine policies that require admin approval (instead of self-release) If you are not using the Preset Policies, you can create a quarantine policy to customize the user experience with quarantined messages. For anything phishing-related, I recommend creating a custom policy that allows the user to “request release from admin.” That means users can raise a hand if they think something should not have been quarantined, and an Incident is created for administrators to review before it is released. To me, this strikes the best balance between security and productivity. This keeps containment intact and gives the SOC final say. It also creates an auditable workflow: who asked for release, who approved it, and why. Bulk Mail Insights: Tune BCL using your tenant’s mail Bulk email (“graymail”) is noisy. Payroll alerts and benefits notifications are legitimate, but they look exactly like phishing. At the same time, true marketing email (graymail) are also bulk. The traditional response (“just whitelist the sender so users stop complaining”) often opens the door for attacker-looking mail to get delivered straight to executives. Defender for Office 365 gives you something better: Bulk Mail Insights (a.k.a. Bulk senders insight). This report shows, over the last 60 days, how much mail at each Bulk Complaint Level (BCL 1–9) was delivered vs. blocked, which senders are generating volume, and where users are likely to experience false positives or false negatives. You can interactively simulate raising or lowering the bulk threshold and immediately see, “If we tighten BCL, how many more messages get quarantined? How many of those were probably junk? How many were probably wanted?” Why this matters: You stop tuning bulk mail based on anecdotes and start tuning based on real telemetry from your own tenant. You can justify decisions to leadership and audit (“We set BCL at X because here is the simulation showing false positive/false negative impact”). You avoid blanket allow rules. Instead, you adjust bulk thresholds for legitimate high-volume senders while keeping stricter actions for everyone else. Note: You can modify the BCL threshold in your default or custom anti-spam policy, but you can’t change it inside the Standard (BCL:6) or Strict (BCL:5) preset security policies themselves. Standard and Strict are already aligned to Microsoft’s recommended baselines. Additional Links: https://security.microsoft.com/senderinsights https://learn.microsoft.com/en-us/defender-office-365/anti-spam-bulk-senders-insight https://learn.microsoft.com/en-us/defender-office-365/mdo-deployment-guide#step-2-configure-threat-policies https://learn.microsoft.com/en-us/defender-office-365/preset-security-policies#policy-settings-in-preset-security-policies https://learn.microsoft.com/en-us/defender-office-365/recommended-settings-for-eop-and-office365 https://www.microsoft.com/en-us/security/blog/2025/07/17/transparency-on-microsoft-defender-for-office-365-email-security-effectiveness/ Anti-Phishing / Impersonation Tuning: Protect the people attackers actually spoof Business email compromise very often looks like this: “Hi, can you handle this payment today?” sent from an address that looks like your CFO or CEO. Microsoft Defender for Office 365 includes targeted impersonation protection, but it only really works if you target your most targeted executives. Here are five pitfalls we see over and over: Empty or stale VIP list Populate “users to protect / high value targets” with executives, finance approvers, legal, anyone authorized to move money or data. Review it monthly. Roles change, and you only get a finite number of protected users (for example, ~350 entries). An out-of-date list silently weakens protection for the people attackers actually impersonate. Phishing email threshold stuck at 1 forever We find organizations that are not using the preset policies have left their phishing threshold values at the default “1” because of initial false positives. We recommend raising it to match the Standard Preset (“3”) or Strict (“4”). Weak action If suspicious “CFO” mail just goes to Junk, users can still act on it. High-confidence impersonation of executives should be quarantined with AdminOnly or request-release workflows, not left in end-user control. Tie this back to the custom quarantine policies (discussed later in this article). Common-name overload If your CEO’s name is something extremely common, you’ll get noise. Expect it. Don’t “turn off” protection for that name — add that address to the Trusted Senders otherwise it will be blocked as an impersonation attempt. Use Trusted Senders / Trusted Domains for known-good partners and vendors so you keep protection high without drowning in alerts. Add only legitimate senders/domains to the Trusted Senders or Trusted Domains instead of lowering enforcement. No scheduled review This control can’t be “set and forget.” Put impersonation tuning and spoof intelligence review on a monthly checklist. That lets you catch new vendors pretending to be finance, new “urgent wire” lure patterns, and any drift from Standard / Strict baseline that Configuration analyzer will also call out. When done right, impersonation protection is not just “spam reduction.” It’s payment fraud prevention. Automated Investigation & Response (AIR): Let Defender remove malicious email before your SOC has to! One of the biggest wins you can land quickly is letting Microsoft Defender for Office 365 automatically remove clusters of malicious messages — without waiting for analyst approval on every single item. Here’s how it works. Defender’s Automated Investigation and Response (AIR) groups messages into “clusters” based on shared indicators like the same malicious URL or malicious file hash. If you opt in to automatic remediation for those cluster types, AIR will go find every matching copy of that threat across the tenant and soft-delete those messages, not just the one that triggered the alert. Why this matters: It turns post-delivery cleanup into something that happens immediately instead of “after Tier 1 has time to review.” It removes known-bad messages from user mailboxes (and related collaboration surfaces like Teams) before a target can click. It dramatically cuts the classic “Did anyone else get this?” manual hunt-and-purge work that burns out SOC analysts. When you configure AIR automation settings in the Microsoft Defender portal (Settings > Email & collaboration > MDO automation settings), you’ll see checkboxes for “Similar files” and “Similar URLs.” Selecting those opts you into automatic soft delete for those clusters. Today, soft delete is the default supported action for these automatic remediations, enabling administrators to undo a deletion, if necessary. This is Defender for Office 365 Plan 2 / Microsoft 365 E5 functionality, and it’s exactly the kind of “secure operations by default” Microsoft has been pushing: detect, contain, and clean up automatically, then let humans investigate with context instead of manually chasing every copy of a phish. This automation triggers when malicious clusters are detected. For automating the classification and triage of user-submitted phishing incidents, check out the Security Copilot Phishing Triage Agent (Preview). Additional Links: GA Announcement: https://techcommunity.microsoft.com/blog/-/auto-remediation-of-malicious-messages-in-automated/4418047 Docs: https://learn.microsoft.com/en-us/defender-office-365/air-auto-remediation Final Thoughts Defender for Office 365 is more than “email filtering.” It’s part of your security operations surface. The decisions you make about automated remediation (AIR), bulk mail thresholds, Safe Links/Attachment behavior, outbound forwarding, connector hygiene, quarantine policy, and impersonation tuning directly determine how easy — or how hard — it is for an attacker to penetrate your organization. Microsoft’s current guidance is clear: Apply Standard or Strict preset security policies so users get the recommended protections by default (for example, Safe Links with no click-through). If you must use a custom policy, review the recommendations from the Configuration analyzer monthly for new recommendations, or to catch and correct drift whenever someone weakens a control. Align internal procedures with the excellent Security Operations Guide for Defender for Office 365. Lock down quarantine so only admins can release high-risk messages, with an auditable “request release” path for users. Turn on automated remediation so Defender can remove malicious clusters of messages before anyone clicks. Organizations that land these basics are in a dramatically better position during an incident. Instead of “Who clicked the link?” you can say, “AIR already pulled it, users were blocked from clicking through, outbound forwarding is disabled, and impersonation of the CFO is quarantined for admin review.” That’s what “secure by default” actually looks like in production. ________ This blog was authored by Joe Stocker, Microsoft Security MVP and Founder of Patriot Consulting Technology Group, in partnership with the Microsoft Defender for Office 365 product team, including Paul Newell, Senior Product Manager, Microsoft Defender for Office 365. Joe Stocker Microsoft Security MVP Learn More and Meet the Author 1) December 16th Ask the Experts Webinar: Microsoft Defender for Office 365 | Ask the Experts: Tips and Tricks (REGISTER HERE) DECEMBER 16, 8 AM US Pacific You’ve watched the latest Microsoft Defender for Office 365 best practices videos and read the blog posts by the esteemed Microsoft Most Valuable Professionals (MVPs). Now bring your toughest questions or unique situations straight to the experts. In this interactive panel discussion, Microsoft MVPs will answer your real-world scenarios, clarify best practices, and highlight practical tips surfaced in the recent series. We’ll kick off with a who’s who and recent blog/video series recap, then dedicate most of the time to your questions across migration, SOC optimization, fine-tuning configuration, Teams protection, and even Microsoft community engagement. Come ready with your questions (or pre-submit here) for the expert Security MVPs on camera, or the Microsoft Defender for Office 365 product team in the chat! REGISTER NOW for 12/16. 2) Additional MVP Tips and Tricks Blogs and Videos in this Four-Part Series: Microsoft Defender for Office 365: Migration & Onboarding by Purav Desai Safeguarding Microsoft Teams with Microsoft Defender for Office 365 by Pierre Thoor You may be right after all! Disputing Submission Responses in Microsoft Defender for Office 365 by Mona Ghadiri (This post) "Microsoft Defender for Office 365: Fine-Tuning" by Joe Stocker Learn and Engage with the Microsoft Security Community Log in and follow this Microsoft Defender for Office 365 blog and follow/post in the Microsoft Defender for Office 365 discussion space. Follow = Click the heart in the upper right when you're logged in 🤍 Learn more about the Microsoft MVP Program. Join the Microsoft Security Community and be notified of upcoming events, product feedback surveys, and more. Get early access to Microsoft Security products and provide feedback to engineers by joining the Microsoft Customer Connection Community. Join the Microsoft Security Community LinkedIn
Safeguarding Microsoft Teams with Microsoft Defender for Office 365
As organizations rely more on Microsoft Teams for daily collaboration, securing this platform has become a top priority. Threat actors are increasingly targeting Teams chats and channels with phishing links and malicious files, making it critical for IT admins and security professionals to extend protection beyond email. Enter Microsoft Defender for Office 365, now armed with dedicated Teams protection capabilities. Microsoft Defender for Office 365 enables users to report suspicious messages, brings time-of-click scanning of URLs and files into Teams conversations, and provides rich alerts and hunting insights for SecOps teams. As a collaborative piece between Pierre Thoor, a Microsoft Security Most Valuable Professional (MVP), and the Defender for Office 365 Product Engineering Team, the below guides with accompanying videos emphasize a proactive, user-driven approach to threat detection and response, turning everyday Teams interactions into actionable security signals for SecOps. See something, say something: Reporting suspicious messages in Microsoft Teams Your fastest sensor isn’t AI – it’s your people. Report this message in Microsoft Teams lets anyone flag a suspicious conversation in two clicks and routes a triageable submission to your security team in the Microsoft Defender portal. Why this matters: Speed to signal: Catch threats at the conversation layer, not just in email. Complete context: Original message, participants, URLs, and verdicts in one place. Habit-forming: A simple, repeatable action employees remember under pressure. How to report (desktop, web, and mobile) In Desktop/Web Hover the message → … More options → Report this message Select Security concern → (optional) add a short note → Report In Mobile (iOS/Android) app Long-press the message → Report message Select Security concern → (optional) add a short note → Report *Tip: Short notes like “Unexpected MFA reset link” help analysts triage faster. Where reports go (for security teams) In the Microsoft Defender portal, navigate to: Investigation & response → Actions and submissions → Submissions → User reported. Open an item to view the Teams message entity (sender/domain, Teams message ID, extracted URLs, verdict) and take action – mark as phish/clean, pivot to Explorer or Advanced Hunting, or copy indicators. Quick setup check Defender portal → Settings → Email & collaboration → User reported settings: enable Monitor reported messages in Microsoft Teams. Licensing: Microsoft Defender for Office 365 Plan 2 (included in Microsoft 365 E5). What good looks like (mini playbook) User reports the message. Security triages the submission and captures the URL/domain and other indicators. Block or allow as appropriate via the Tenant Allow/Block List (TABL). Hunt for related activity or clicks (see Video 3). Close the loop: thank the reporter and share the outcome to reinforce the behavior. Common gotchas Reporting is disabled in the Teams messaging policy – verify before rollout. Some users assume “Report” notifies the sender – clarify that it routes to the Security team, not the sender. Call to action: Enable reporting for your users and add this line to your awareness site: “If it feels phishy, report – don’t click.” Think before you click - Safe Links catches threats at click-time Links can change after delivery. Safe Links waits until click-time, evaluates the destination, and shows an in-app warning page in Teams. Pair it with the Tenant Allow/Block List (TABL) to tune quickly across the tenant. Why this matters Prevents delayed redirects: Avoids “clean-at-send” methods. Consistent protection in Teams: Familiar warning UX reduces risky clicks. Rapid tuning: Block newly observed domains in seconds; no advanced transport rules required. What you’ll see in the video Policy check (Teams in scope) Defender portal → Email & collaboration → Policies & rules → Threat policies → Safe Links → ensure Apply Safe Links to Microsoft Teams is enabled for target users or groups OR that you use Standard/Strict Preset Policy. Warning page at click-time Post a benign test URL in Teams and click it to show the Safe Links warning experience. Block it as you spot it (Allow/Block) Defender portal → Threat policies → Tenant Allow/Block List → URLs → Add (domain or URL). Re-click in Teams – now blocked at click-time. Optional telemetry (Advanced Hunting) Confirm outcomes and adoption: UrlClickEvents | where Timestamp > ago(24h) and Workload == "Teams" | summarize Clicks=count(), Users=dcount(AccountUpn) by ActionType | order by Clicks desc Deployment tips Start with a pilot group that includes IT + power users; expand after validation. Create a review cadence for TABL (e.g., monthly) and expire temporary blocks. Troubleshooting No warning page? Verify policy scope includes the user and the Teams workload. Block not taking effect? Give TABL a short sync window, then re-test; confirm you blocked the correct domain/URL pattern. “Hunt the chat”: Advanced hunting for Teams threats Overview With Advanced Hunting you can quickly reconstruct activity in Microsoft Teams – who sent the message, who clicked the link, and what protections kicked in. This section shows how the four Teams-relevant tables work together, so you can move from signal to action quickly. New: message warnings for malicious URLs (internal and external) Teams now shows a warning banner on messages that contain URLs flagged as spam, phishing, or malware. Warnings appear in internal and external chats/channels, and can be added after delivery (up to ~48 hours) if a URL’s reputation changes. This complements Safe Links (time-of-click) and doesn’t replace ZAP; when ZAP removes a message, that action takes precedence. Public preview began September 2025; GA November 2025, enabled by default at GA and manageable in Teams admin center → Messaging settings. See Message Center: https://admin.cloud.microsoft/?#/MessageCenter/:/messages/MC1150984 The four tables you’ll use MessageEvents – delivery context (sender, thread, internal vs. external). MessagePostDeliveryEvents – post-delivery actions, including Phish ZAP and Malware ZAP. MessageUrlInfo – URLs extracted from Teams messages. UrlClickEvents – time-of-click outcomes for links, including those clicked in Teams. What you’ll learn in the video Surface active external domains in your tenant’s Teams chats. Identify who clicked risky links and the click outcomes (via Safe Links telemetry). See where message warnings appear in the chat UI. Pivot to an incident and block indicators fast via the Tenant Allow/Block List (TABL). A couple hunts to try right now 1) Malicious verdicts in Teams (last 24 hours) Find messages that already carry a Spam/Phish/Malware verdict – your fastest triage queue. MessageEvents | where Timestamp > ago(1d) | where ThreatTypes has "Phish" or ThreatTypes has "Malware" or ThreatTypes has "Spam" | project Timestamp, SenderDisplayName, SenderEmailAddress, RecipientDetails, IsOwnedThread, ThreadType, IsExternalThread, ReportId Use it for: a quick sweep + pivot to incident/entities, then TABL block if needed. 2) “IT helpdesk” imposters in external DMs (last 5 days) Surface social-engineering lures that impersonate support. MessageEvents | where Timestamp > ago(5d) | where IsExternalThread == true | where (RecipientDetails has "help" and RecipientDetails has "desk") or (RecipientDetails has "it" and RecipientDetails has "support") or (RecipientDetails has "working" and RecipientDetails has "home") or (SenderDisplayName has "help" and SenderDisplayName has "desk") or (SenderDisplayName has "it" and SenderDisplayName has "support") or (SenderDisplayName has "working" and SenderDisplayName has "home") | project Timestamp, SenderDisplayName, SenderEmailAddress, RecipientDetails, IsOwnedThread, ThreadType, ReportId Use it for: first-contact scams (external tenant posing as IT). Pair with Safe Links telemetry to see who clicked. Tip: has is token-aware and generally faster/cleaner than contains for word matches. Keep both hunts detection-ready by ensuring the final projection includes Timestamp and ReportId. 3) BONUS! External DMs with links (last 7 days) MessageEvents | where Timestamp > ago(7d) and IsExternalThread == true | join kind=inner (MessageUrlInfo) on TeamsMessageId | summarize Links=dcount(Url), Senders=dcount(SenderEmailAddress) by UrlDomain | top 10 by Links desc 4) Who clicked (Teams workload) – exposure view: UrlClickEvents | where Timestamp > ago(7d) and Workload == "Teams" | project Timestamp, AccountUpn, Url, ActionType | order by Timestamp desc “From Hunt to Action”: Respond & contain Finding a risky link in Teams is only half the job. This walkthrough shows how to go from detection to containment – block the domain, clean up delivered messages, and cut attacker access. Why this matters Speed: Shrink time from “we saw it” to “it’s blocked”. Consistency: Turns ad-hoc hunting into a repeatable response flow. Coverage: Pair URL blocking with identity and device containment. What you’ll see in the video Turn a hunt into an alert In Advanced Hunting, run a short query (below) and choose Create detection rule to schedule it. Alerts auto-create incidents you can triage. Block at click-time (Safe Links + TABL) In the incident, open the URL entity and add the URL/domain to the Tenant Allow/Block List (TABL) so future Teams clicks are blocked by Safe Links. Post-delivery cleanup (ZAP) If a malicious message slipped through, ZAP can remove or mark it after delivery. You’ll see evidence on the incident timeline. Contain accounts and devices Revoke user sessions in Entra ID to invalidate active tokens. Reset the password (and require strong, unique credentials), then enforce MFA for the account. Review MFA methods and remove anything suspicious; review app consents and revoke illicit grants. If endpoints are onboarded, isolate the device in Microsoft Defender for Endpoint to stop outbound connections while you investigate. The Microsoft Learn guide, https://learn.microsoft.com/en-us/defender-office-365/responding-to-a-compromised-email-account, for compromised accounts recommends session revocation, password reset, MFA enforcement, reviewing OAuth app consents and admin roles, and checking mail forwarding/rules – steps that complement the Teams response you see here. The hunt This KQL surfaces rare external domains in Teams and any user clicks. let lookback = 1d; // External Teams messages let externalMsgs = MessageEvents | where Timestamp > ago(lookback) and IsExternalThread == true | project MsgTime = Timestamp, TeamsMessageId, SenderEmailAddress, ME_ReportId = ReportId; // URLs found in Teams messages let urlsInMsgs = MessageUrlInfo | where Timestamp > ago(lookback) | project MUI_Time = Timestamp, TeamsMessageId, Url, UrlDomain, MUI_ReportId = ReportId; // Clicks coming from Teams let clicks = UrlClickEvents | where Timestamp > ago(lookback) and Workload == "Teams" | project ClickTime = Timestamp, Url, Clicker = AccountUpn, ClickAction = ActionType, UCE_ReportId = ReportId; // Define “rare” domains in the period let rareDomains = urlsInMsgs | summarize msgCount = dcount(TeamsMessageId) by UrlDomain | where msgCount < 3; rareDomains | join kind=inner (urlsInMsgs) on UrlDomain | join kind=leftouter (externalMsgs) on TeamsMessageId | join kind=leftouter (clicks) on Url | project Timestamp = coalesce(ClickTime, MUI_Time, MsgTime), UrlDomain, Url, SenderEmailAddress, Clicker, ClickTime, ClickAction, TeamsMessageId, ReportId = coalesce(UCE_ReportId, MUI_ReportId, ME_ReportId) After verifying results, select Create detection rule, set a schedule (e.g., hourly), and map entities so incidents include the right artifacts. What good looks like (response playbook) Alert fires → open incident; confirm scope and entities. Block URL/domain via TABL to stop future clicks. Confirm ZAP removed or marked delivered messages. Revoke sessions and reset password; enforce MFA. Review MFA methods and remove unknown devices/methods. Audit app consents (revoke illicit grants) and verify the user holds no unexpected admin roles. If email abuse is suspected, check for forwarding or malicious Inbox rules. Isolate device if execution is suspected; collect artifacts and un-isolate after remediation. FAQs Does the block remove the message? No – TABL blocks at click-time. Post-delivery removal is handled by ZAP when detections apply. Will revoking sessions disrupt users? It forces sign-in again (expected). Communicate this in your response template. What if the attacker used consent phishing? Revoke the offending enterprise app consent and review publisher verification status. Call to action: Save the query, create the detection, and attach this playbook to your incident template. The goal every time: find → block → clean up → contain Securing Microsoft Teams is most effective when technology and people work together. By enabling user reporting, leveraging real-time protections, and empowering security teams to act quickly, organizations can turn everyday collaboration into a strong defense against threats. ## Please take two minutes to take this survey to let us know what you think of this blog (series), video, and community content. Questions or comments on this blog "Microsoft Defender for Office 365 – A Four-Part Guide to Secure Collaboration" for the author or other readers? Please log in and post your response below! _____________ This blog has been generously and expertly authored by Microsoft Security MVP, Pierre Thoor with support of the Microsoft Defender for Office 365 product team. Pierre Thoor Microsoft Security MVP | Microsoft Defender for Office 365 Champ Learn More and Meet the Author 1) December 16th Ask the Experts Webinar: Microsoft Defender for Office 365 | Ask the Experts: Tips and Tricks (REGISTER HERE) DECEMBER 16, 8 AM US Pacific You’ve watched the latest Microsoft Defender for Office 365 best practices videos and read the blog posts by the esteemed Microsoft Most Valuable Professionals (MVPs), now bring your toughest questions or unique situations straight to the experts. In this interactive panel discussion, Microsoft MVPs will answer your real world scenarios, clarify best practices, and highlight practical tips surfaced in the recent series. We’ll kick off with a who’s who and recent blog/video series recap, then dedicate most of the time to your questions across migration, SOC optimization, fine-tuning configuration, Teams protection, and even Microsoft community engagement. Come ready with your questions (or pre-submit here) for the expert Security MVPs on camera, or the Microsoft Defender for Office 365 product team in the chat! REGISTER NOW for 12/16. 2) Additional MVP-Authored Blogs in this Four- Part Series: Microsoft Defender for Office 365: Migration & Onboarding by Purav Desai (This post) Safeguarding Microsoft Teams with Microsoft Defender for Office 365 You may be right after all! Disputing Submission Responses in Microsoft Defender for Office 365 by Mona Ghadiri Microsoft Defender for Office 365: Fine-Tuning by Joe Stocker Learn and Engage with the Microsoft Security Community Log in and follow this Microsoft Defender for Office 365 blog and follow/post in the Microsoft Defender for Office discussion space. Follow = Click the heart in the upper right when you're logged in 🤍 Learn more about the Microsoft MVP Program. Join the Microsoft Security Community and be notified of upcoming events, product feedback surveys, and more. Get early access to Microsoft Security products and provide feedback to engineers by joining the Microsoft Customer Connection Community. Join the Microsoft Security Community LinkedInIntroducing the Microsoft Defender for Office 365 ICES vendor ecosystem
In today's digital landscape, the need for comprehensive security measures is more critical than ever, as email continues to be a primary vector for cyberattacks such as phishing and malware. To address this, Microsoft Defender for Office 365 leverages the extensive scale of Microsoft's threat intelligence, which processes trillions of signals daily. By integrating Large Language Models (LLMs) and advanced Natural Language Processing, Defender for Office 365 empowers organizations with AI-driven threat detection, behavioral analytics, and automated responses thus proactively identifying and neutralizing risks before they reach end users. This collaborative defense approach reinforces the principle that security is a team sport, requiring shared intelligence and coordinated action across the ecosystem. We recognize in today’s dynamic cyber threat landscape, defense-in-depth strategy has become a vital approach not only for Microsoft customers but also across the broader Secure Email Gateway (SEG) market. Organizations are increasingly adopting layered security solutions to comply with regulatory requirements, enhanced detection, and ensure robust protection. To address this, we’re announcing the Microsoft Defender for Office 365 ICES Vendor Ecosystem — a unified framework that enables seamless integration with trusted third-party vendors. This ecosystem is designed to eliminate integration friction and deliver: Broader detection coverage through vendor diversity Transparency across Microsoft Defender for Office 365 and partner detections Streamlined SOC workflows through consistent policy enforcement and shared investigation tools Stronger compliance alignment with layered security mandates This partner ecosystem is about creating a cohesive defense fabric that enhances SOC efficiency with Microsoft Defender for Office 365 as the foundation. The ecosystem also provides flexibility, scalability, and preparedness for the complexities of contemporary enterprise security. With this in mind, we are pleased to announce that our trusted ICES security vendors, Darktrace and KnowBe4, have become the first launch partners within our ecosystem. They offer customers a seamless and collaborative defense framework where each solution enhances the strengths of the others. We welcome additional partners soon as we continue to expand this integrated ecosystem. “Our integration with Microsoft gives security teams the tools they need to act faster and more precisely to detect and respond to threats,” said Jill Popelka, CEO of Darktrace. “Together, we’re strengthening defenses where it matters most to our customers —at the inbox.” “I’m incredibly excited at the opportunity afforded by this partnership with Microsoft and the deeper integrations it enables. Leveraging this integration allows us to use our vast quorum of data around email security and human risk in a way that provides the most comprehensive layered security approach available to the market. A complementary defense strategy is mandatory and this integration with Microsoft M365 furthers that vision by combining our capabilities to create comprehensive defense strategies that address the full spectrum of modern cyber threats.” noted Greg Kras, Chief Product Officer @ KnowBe4 Unified Quarantine The core strength of this new ecosystem is the seamless integration between Defender for Office 365 and its ICES partners, through the Unified Quarantine feature. Managing quarantined messages from multiple solutions can often be complex and inefficient. Unified Quarantine streamlines the process by consolidating quarantined items identified by both Defender for Office 365 and third-party (3P) solutions into a single, unified interface, enhancing customer ease and visibility. Administrators can efficiently review, release, or remediate messages through this unified interface, irrespective of the provider that identified the threat. This approach not only optimizes time management but also guarantees uniform policy enforcement and facilitates transparency on detections, resulting in improved operational efficiency and a more coherent user experience. As part of the Unified Quarantine, security admins can also see which provider quarantined the message. Transparency and Insight Across Solutions In environments with multiple email security solutions, transparency is crucial to understanding each vendor's detections. Microsoft Defender for Office 365 offers a unified dashboard that clearly distinguishes between threats stopped by Defender and those identified by third-party solutions, ensuring transparent and fair attribution of protection value. This dashboard provides security teams with a comprehensive view of how each solution contributes to protection, helping to identify overlapping coverage and areas of unique value. This clarity supports more informed decision-making around threat trends, policy optimization, and vendor strategy fostering stronger collaboration between internal teams and external partners. Deeper SOC Investigation Capabilities: Threat Explorer, Advanced Hunting, and Email Entity Page Modern defenders need tools for rapid investigation, root cause analysis, and tactical response. The Defender for Office 365 ecosystem unifies investigative workflows across partner solutions. Within Threat Explorer, security analysts can seamlessly pivot between messages actioned by Microsoft Defender for Office 365 and those flagged by integrated partners. The side-by-side display of verdicts and actions enables quick correlation and pattern recognition. Advanced Hunting brings even greater depth, allowing analysts to craft queries that span both Microsoft Defender for Office 365S and 3P data sources. This holistic view accelerates threat hunting and helps organizations surface novel attack techniques or gaps in coverage. EmailEvents | where Timestamp > ago(7d) //List emails caught by a Third-party solution | where DetectionMethods contains "Thirdparty" | project NetworkMessageId, RecipientEmailAddress, ThreatTypes, DetectionMethods, AdditionalFields, LatestDeliveryLocation On the Email Entity Page, every message surfaces a complete action history, including which product took action and what verdict was assigned. This granular visibility demystifies complex incidents and builds confidence in the layered defense model. Summary As the threat landscape continues to evolve, so must our defenses. While organizations embrace defense-in-depth, fragmented integrations may lead to unintended consequences such as diminished detection capabilities, overlapping controls, and SOC inefficiencies. With the Defender for Office 365 ICES vendor ecosystem, Microsoft is setting a new standard for collaborative, integrated security platforms. By combining proven protection, seamless partnerships, and unified visibility, organizations can embrace defense-in-depth without complexity or compromise. Whether combating phishing, malware, or the next generation of email-borne threats, customers benefit from a defense-in-depth strategy built for agility and efficiency. With hands-off enablement, unified experiences, and unmatched transparency, the Defender for Office 365 ecosystem empowers every organization to stay one step ahead—today and tomorrow. Learn More To learn more about the Microsoft Defender for Office 365 ICES Vendor Ecosystem, please visit https://learn.microsoft.com/defender-office-365/mdo-ices-vendor-ecosystem.Protect your organizations against QR code phishing with Defender for Office 365
QR code phishing campaigns have most recently become the fastest growing type of email-based attack. These types of attacks are growing and embed QR code images linked to malicious content directly into the email body, to evade detection. They often entice unwitting users with seemingly genuine prompts, like a password reset or a two-factor authentication request. Microsoft Defender for Office 365 is continuously adapting as threat actors evolve their methodologies. In this blog post we’ll share more details on how we’re helping defenders address this threat and keeping end-users safe.Microsoft Defender EOP
We have been experiencing an issue since last week where we are unable to view the details of quarantined emails. Could you please confirm if this is related to a known backend service issue, or if there are any specific troubleshooting steps we should perform on our end? Any guidance or updates would be greatly appreciated.
Microsoft Defender for Office 365's Language AI for Phish: Enhancing Email Security
Email security presents a complex challenge for individuals and organizations alike. Over the years, attackers have evolved from simple spam campaigns to sophisticated threats including ransomware, identity theft schemes, and carefully crafted phishing scams. Now, malicious actors are armed with Generative AI and are advancing at an alarming pace. In response, Microsoft Defender for Office 365 has dedicated extensive research and development efforts to making email security smarter, more flexible, and more proactive. This dedication led to the introduction of specialized language intelligence to fight Business Email Compromise (BEC) attacks, announced last year at Ignite 2024 (Microsoft Ignite: Redefining email security with LLMs to tackle a new era of social engineering | Microsoft Community Hub). With that announcement, we offered a significant leap in analyzing suspicious messages using advanced natural language processing, enabling organizations to better detect subtle manipulative emails designed to lure unsuspecting users into revealing confidential data or transferring funds. The threats, however, have not stopped with BEC. Phishing attacks are constantly evolving, leveraging new tactics and forms. As part of Defender for Office’s mission to stay one step ahead of these threats, we’re taking the same robust Language AI approach we used for BEC analysis and applying it to a broader spectrum of phishing attacks. Today we’re excited to announce Microsoft Defender for Office 365’s new Language AI for Phish model. This model progressively learns from thousands of real-world phishing attempts and analyzes all messages classified as phish. Furthermore, it incorporates advanced Machine Learning and Natural Language Processing (NLP) techniques to read, process, and understand email content the way a human analyst might, yet in a fraction of the time and at an immense scale. Our model has been operational since April 2025, achieving over 99.99% accuracy and blocking 1 million phishing emails daily. By advancing our language AI and rigorously training it on phishing email threats, we are further strengthening the comprehensive protections established by our BEC-focused innovations. These enhanced capabilities create an integrated security framework designed to proactively address evolving risks and accelerate response times to emerging threats. Through Microsoft Defender for Office 365’s commitment to continuous improvement, this expanded approach empowers organizations and individuals to maintain a strong security posture in the face of ever-changing cyber challenges. Learn More: To learn more about Microsoft Defender for Office 365’s Language AI capabilities, please read more here or visit our website.
