Outlook Cached Mode Repeatedly Re-syncs Mailbox After Restart (Starts Again Around 3.99 GB)
Hi everyone, I’m experiencing a strange Outlook Cached Exchange Mode issue with a Microsoft 365 mailbox after a recent Windows rebuild and wanted to see if anyone has seen similar behavior. Environment: Microsoft 365 mailbox (Exchange Online) Outlook for Microsoft 365 Version 2605 Build 16.0.20026.20076 64-bit Windows 11 25H2 Fresh OS rebuild performed twice New Outlook profile created Office completely reinstalled OST recreated multiple times Issue: When Cached Exchange Mode is enabled, Outlook starts downloading/synchronizing mail normally, and the OST file continues to grow correctly. However, after every reboot or Outlook restart, Outlook again shows “Downloading…” starting from around 3.99 GB. Important observations: Online Mode works perfectly OUTLOOK.EXE closes properly after exit OST file is NOT deleted and continues growing Sync slider changes (1 month, 1 year, all mail) make no difference Disabling Outlook indexing did not help New Outlook profile did not help Reinstalling Office did not help Problem only started after OS rebuild Before rebuild, same mailbox worked normally in Cached Mode No pending office or windows update. It does not appear to actually re-download the mailbox from scratch because the OST size keeps increasing, but Outlook repeatedly processes/downloads from around the same 3.99 GB point after restart. Has anyone seen: Cached Mode replaying synchronization repeatedly after restart? Similar behavior on recent Current Channel builds? Security/EDR products interfering with OST synchronization state? Any known regressions with Outlook Version 2605 Build 16.0.20026.20076? Any suggestions or similar experiences would be appreciated.
Migration from Hosted Exchange (Hybrid) to M365 Classic Outlook Client Problems and Solutions
Hello Everyone, I'm a tech who started on a 8088 processor in the 80's. Not mentioning the Vic20 and C64 since that hardly seem relevant! I'm posting here to hopefully help the next person with the issues I've had over the last few weeks. My client had to port his email from a provider with an on-perm Exchange server in a Hybrid setup with M365 to his own M365 environment. I expected this was to be about 3 hours of work for me - setup M365 environment, plan the cut-over window, update the Outlook clients on each PC. It ended up being roughly 20 hours of my time and at least 10 hours of dedicated time for my client. For those wanting to jump directly to what mostly fixed it use this link, it should get you past the dreaded "an encrypted connection to your mail server is not available" when trying to add the mail account into a clean profile. Use https://support.microsoft.com/en-us/windows/classic-outlook-troubleshooters-086e3d66-5404-4034-9cc5-545909dcc166 and pick "Classic Outlook Profile Setup Troubleshooter" Most hits are going to tell you its an autodiscovery issue, but if you're reading this I'm going to assume you've already confirmed that. Our issue was some ghost configuration, only on the PCs previously setup for mail on the old server. A new PC could add the same account without issue. Some of the research suggested this would not happen if the proper Microsoft migration process is followed to move the account - but in our case the previous provider was unable to perform the migration. I'll skip over the research we tried along the way, such as New Outlook Profiles, Registry entry changes, MS Personal users with the same email as MS Business Users, Autodiscover problems (including concerns that the base website for the client was offering invalid data), and so on. After each hit where we applied a fix we again had to try adding the mail to the profile, and each time we sat watching the little circle for up to 5 minutes only to get the same error. Now, once we found the link above - which did not come up in most searches - things got better, but not 100%. We added the profile ok but then Outlook gave a permission error while starting. To fix that, the user signed in must have administrative access and you use File Explorer to navigate to the folder identified in the error. In our case it was in folders kept under \Windows\System32\. When prompted that we need to grant permanent access we said yes. In our case this is where Outlook was storing the ost files. That worked for most of the clients, but we had one additional issue where the error was pointing to a folder that didn't exist. Just creating the folder was not enough, the final fix was to hold CTRL-SHIFT down while opening Outlook to start in administrative mode to allow it to create the ost file in the newly created folder. Finally 3 weeks after our cut over window, while the client had to use OWA, we were able to get outlook running. This was critical for my client because they did not have access to the mail history since the migration didn't happen - they had to open a copy of their PST in Outlook and use mail in OWA and constantly bounce back and forth. I hope this helps someone avoid the pain we went though!Microsoft Rolls Out the New Calendar Sharing Model
An automatic upgrade is currently ongoing to move Outlook calendar sharing from the old MAPI-based model to a new REST-based model. The new model has been available to Outlook classic clients on an optional basis and is now becoming the default. The REST model is simpler and exploits the Exchange Online service instead of depending on client transactions. It’s also part of the transition to the new Outlook. https://office365itpros.com/2026/05/28/calendar-sharing-outlook/Microsoft 365 Outlook Classic Latest Update Copilot issues
Hello to all, we are currently using the Microsoft 365 Outlook Classic, using version Version 2604 Build 16.0.19929.20172 (Current Channel) Copilot works as expected; However ever since the next update was released (Version 2605 (Build 20026.20076)) some users including me, have been getting the below error: I have tried uninstalling and installing the latest version directly, with the same issue. Scanned for any corrupted system files with no issues,but still get the error; I have tried the update Licence from the Outlook application, with no issues but the copilot something went wrong error keeps on persisting. Internet searches are very vague and the I have found some registry suggestions which I have also tried with no solution. The only way to get Copilot working on Outlook Classic was to revert to a previous working version, only then Copilot works as expected. This only happens in Outlook Classic; Copilot works fine on the latest version of Microsoft Office 365 with Word, Excel, etc. For those that maybe might ask why not switch to the New Outlook, we have had issues with it and it has been recommended to use the Classic Outlook for the time being. Has anyone been experiencing such issues please? Thanks for any help.HVE for Microsoft 365: When to Use It, When Not To, and Who Should Be Allowed to Send at Scale
Microsoft recently announced the General Availability of High Volume Email for Microsoft 365, also known as HVE, in Exchange Online. This is an important and long-awaited capability for organizations that need to send large volumes of internal email from applications, devices, or line-of-business systems without using regular user mailboxes as bulk-sending engines. But HVE should not be misunderstood. It does not mean that every mailbox in Exchange Online should now be used for mass email. It does not mean Exchange Online has become a general-purpose marketing platform. And it does not remove the need for proper outbound email governance. Why HVE Matters For years, many organizations have used regular Exchange Online mailboxes, shared mailboxes, or service accounts to send automated messages from applications, scanners, monitoring platforms, ticketing platforms, and custom business applications. That approach creates several problems. Standard mailboxes are designed for human and business communication, not for sustained high-volume automated traffic. Exchange Online has recipient limits, message rate limits, outbound spam protections, and tenant-level controls to protect the service and reduce abuse. HVE introduces a more appropriate model for specific high-volume scenarios. Instead of using a normal mailbox for automated traffic, organizations can create dedicated HVE accounts and use specific SMTP endpoints, admin controls, reporting, and governance for approved high-volume internal messaging scenarios. What HVE Is Designed For HVE is designed for automated, operational, and transactional messaging at scale, primarily for internal recipients within the tenant. Examples include: Internal application notifications. Line-of-business system messages. Device-generated messages. Operational alerts. Security advisories. Internal workflow communications. Monitoring platform alerts. IT service notifications. Large-scale internal announcements generated by systems. This is especially relevant when the organization needs to send messages at scale but still wants to keep the workload within Microsoft 365 governance and Exchange Online mail flow. In practical terms, HVE is useful when the sender is not a human user, but a controlled business system. What HVE Is Not HVE is not a replacement for marketing platforms. HVE is not a general-purpose internet bulk email engine. HVE is not a way to bypass Exchange Online sending limits for external campaigns. HVE is not the correct platform for newsletters, promotional campaigns, large-scale customer communication, or high-volume external transactional email. For external transactional, marketing, or customer-facing bulk email, organizations should evaluate platforms designed for that purpose, such as Azure Communication Services Email, SendGrid, Amazon SES, Mailchimp, Brevo, or another specialized delivery platform. When to Use HVE Use HVE when the workload matches these characteristics: The sender is an application, device, service, or business system. The recipients are primarily internal users in the Microsoft 365 tenant. The volume is higher than what should be sent from a standard mailbox. The workload is operational, automated, or transactional. The organization needs centralized Microsoft 365 administration and reporting. The organization wants to avoid impacting user mailbox sending limits. The use case is approved, documented, monitored, and governed. Good examples: A security platform sending internal security advisories. A monitoring system sending infrastructure alerts to internal teams. A business workflow system sending high-volume approval or status notifications. An IT service platform sending internal notifications. A service management platform sending ticket updates to internal users. A device management system sending operational messages to internal teams. When Not to Use HVE Do not use HVE when the workload is external bulk email. Avoid HVE for: Marketing campaigns. Newsletters to customers. Promotional email. Mass external invitations. External transactional email at scale. Customer invoices and receipts in high volume. OTP or password reset flows for external users. External portal notifications. Any workload where deliverability, bounce handling, reputation management, unsubscribe handling, analytics, or customer consent management are required. Those workloads require a platform designed for external delivery, reputation management, suppression lists, opt-out, tracking, bounce handling, and compliance. Who Should Be Allowed to Use HVE HVE should not be enabled casually for every team or every application. It should be treated as a controlled platform capability. Recommended eligible senders: Approved line-of-business applications. Corporate systems owned by IT, Security, Operations, Facilities, or Service Management teams. Managed devices or services with a clear business purpose. Internal platforms that send operational messages to employees. Applications with documented ownership, authentication, monitoring, and expected volume. Recommended non-eligible senders: Normal users. Shared mailboxes used by humans. Marketing teams sending to external audiences. Unmanaged scripts. Legacy systems with no owner. Applications with unknown volume. Systems that send to external recipients at scale. Any application using HVE just to avoid standard mailbox limits. The core principle is simple: HVE should be enabled for workloads, not for convenience. Governance Model Before enabling HVE, organizations should define a governance model. At minimum, each HVE account should have: A named business owner. A technical owner. A documented purpose. Expected daily and monthly volume. Recipient scope. Authentication method. Monitoring process. Incident response path. Decommissioning criteria. Review frequency. HVE accounts should not become invisible service accounts that nobody owns. They should be treated as privileged communication identities. Security and Authentication HVE supports OAuth authentication, and Microsoft provides guidance for restricting OAuth authentication to specific Microsoft Entra ID applications. This is important because organizations should avoid broad, uncontrolled access. They should restrict which applications can send through each HVE account, monitor usage, and separate workloads by purpose. For example: One HVE account for security alerts. One HVE account for monitoring systems. One HVE account for IT service notifications. One HVE account for internal operational communications. This separation improves visibility, investigation, accountability, and risk containment. HVE vs Standard Exchange Online Mailboxes A standard Exchange Online mailbox should be used for normal human communication. A shared mailbox should be used for collaborative business processes. An HVE account should be used for approved high-volume internal system email. A dedicated external delivery platform should be used for marketing, bulk external communication, or high-volume transactional email. Scenario Recommended Platform Human business email Exchange Online mailbox Team or department mailbox Shared mailbox Low-volume application notifications Standard Exchange Online, if approved High-volume internal system notifications HVE Internal operational alerts at scale HVE Marketing campaigns Marketing platform External transactional email Transactional email service Customer newsletters Marketing automation platform OTP/password reset for external users Dedicated transactional platform External bulk email Dedicated bulk email provider HVE and the Mailbox External Recipient Rate Limit Cancellation Microsoft also announced that the Mailbox External Recipient Rate Limit in Exchange Online was cancelled indefinitely. However, that cancellation should not be interpreted as permission to use Exchange Online for uncontrolled bulk sending. Microsoft was clear that other limits remain unchanged, including the existing Recipient Rate Limit and the Tenant-level External Recipient Rate Limit. That distinction is important. The cancellation of one mailbox-level external recipient limit does not remove the need for proper architecture. Exchange Online still has service limits. Outbound spam controls still apply. Tenant-level protections still matter. And HVE is still not a marketing engine. Practical Architecture Decision Before enabling HVE, ask these questions: Who is sending? Is the sender a human, shared mailbox, application, or device? Who are the recipients? Are they internal or external? What is the expected volume? Is the workload operational, transactional, promotional, or human communication? Does the business need Microsoft 365 mail flow and governance? Does the use case require bounce handling, unsubscribe, tracking, or reputation management? Is the application properly authenticated and monitored? Who owns the account? Who approves the sending pattern? Who responds if the account is abused? If the workload is internal, automated, high-volume, and business-approved, HVE may be the right answer. If the workload is external, promotional, customer-facing, or marketing-driven, use a dedicated email delivery platform. Recommended Enablement Approach Organizations should enable HVE in phases. First, identify existing systems currently using user mailboxes, shared mailboxes, or SMTP AUTH for automated sending. Second, classify each workload as internal, external, operational, transactional, marketing, or human communication. Third, migrate only approved internal high-volume workloads to HVE. Fourth, move external high-volume workloads to dedicated email delivery platforms. Fifth, monitor usage and review HVE accounts regularly. This avoids turning HVE into another uncontrolled sending layer. Conclusion High Volume Email for Microsoft 365 is an important addition to Exchange Online. It gives organizations a native way to support high-volume internal system messaging without using standard mailboxes for automated high-volume traffic. But HVE is not a free pass for bulk email. It is not a marketing platform. It is not a replacement for transactional email services. And it should not be enabled for every mailbox or every application. The right approach is workload classification. Use Exchange Online for corporate communication. Use HVE for approved high-volume internal system messaging. Use dedicated platforms for external bulk, marketing, and transactional email. The question is not only: “Can this system send email through Microsoft 365?” The better architectural question is: “What type of email is this, who is the audience, and what is the correct platform for this workload?” That is where proper email architecture begins.
