Exchange Outbound Email Delivery Failures - DNS Resolution Issue (Error 450 4.4.312)

Hi,

Microsoft Exchange Online/Office 365 servers are failing to deliver emails to our mail server due to DNS resolution failures. This issue started approximately one week ago with no configuration changes on our end. All other email being delivered without problems. Any help or suggestions gratefully received 🙏

Technical Details

Error Code: 450 4.4.312 DNS query failed [Message=ErrorRetry]

Key Finding: Microsoft's Exchange servers are failing to resolve our domains when attempting email delivery, returning 0.0.0.0 during DNS lookups.

Affected Domains:

• iteracy.com
• smartfixuk.com
• telexsus.com
• Multiple other domains hosted on our server's IP 148.253.141.57

Sample Error from NDR (sent to Microsoft users):

Server at iteracy.com (0.0.0.0) returned '450 4.4.312 DNS query failed [Message=ErrorRetry] [LastAttemptedServerName=iteracy.com]'

Email Flow Issue

Problem: Microsoft users sending emails to our domains (e.g., mailto:email address removed for privacy reasons → mailto:email address removed for privacy reasons) are getting delivery failures because Microsoft's outbound mail servers cannot resolve our domain's mail server.

DNS Configuration (All Working Correctly)

Domain: iteracy.com
MX Record: mail.iteracy.com (priority 0)
A Record: mail.iteracy.com → 148.253.141.57
PTR Record: 148.253.141.57 → server.iteracy.com
Nameservers: ns1.iteracy.com, ns2.iteracy.com (both → 148.253.141.57)

Extensive Testing Performed

DNS Providers That Resolve Our Domains Correctly:

• ✅ Google DNS (8.8.8.8)
• ✅ Cloudflare (1.1.1.1)
• ✅ Multiple global DNS propagation checkers

Microsoft Exchange Online Specific Issues:

• ❌ Microsoft's outbound mail servers return 0.0.0.0 when looking up our domains
• ❌ Seemingly only Microsoft's mail infrastructure affected
• ❌ Multiple domains on same server IP affected
• ❌ Email delivery from any Microsoft system (Office 365, Outlook.com) to our domains fails

What We've Ruled Out

1. IP Blacklisting: Checked Microsoft's delisting portal (sender.office.com) - IP not blocked
2. DNS Configuration: All records correct and resolving globally on all other DNS providers
3. Recent Changes: No DNS, hosting, or configuration changes made
4. Reverse DNS: PTR records properly configured and resolving correctly

Business Impact

• Complete email isolation from Microsoft ecosystem (Office 365, Outlook.com, Hotmail, etc.)
• Microsoft users cannot send emails to our domains
• Client complaints and business disruption
• Messages temporarily queued but will eventually bounce if not resolved

Support Experience

Microsoft front-line support claims "not our issue" despite evidence that only Microsoft's outbound mail servers are failing DNS resolution for our domains.