Azure communication services emails don't deliver to some email addresses

Question

We have Azure Communication Services configured to send emails from a domain. The domain has seemingly been fully verified. Emails deliver immediately to users who have gmail accounts, but users who access emails via outlook don't get our emails. There are no clear errors in the logs to tell us why the emails are not being delivered, and they aren't visible in spam folders.

Answer 1

Hi @Ryan

Thank you for reaching us regarding the issue.

Azure Communication Services (ACS) emails show as "Succeeded" or "OutForDelivery" after handoff to its transport, but downstream delivery to Outlook/Microsoft-hosted inboxes can still fail silently or be filtered.
ACS provides limited visibility into recipient-side issues (e.g., no detailed NDRs for all cases). Gmail often accepts messages more leniently, while Microsoft 365/Outlook.com/Exchange Online applies stricter filtering, anti-spam policies, and authentication checks.

Key Documentation References

• Domain & Sender Authentication: Ensure full verification of your custom domain (Domain + SPF + DKIM + DKIM2). SPF must use -all (hard fail), not ~all.
• Email Statuses: "Succeeded" means the send operation completed and the email is out for delivery. Further status (Delivered, Failed, Bounced, FilteredSpam, Quarantined, Suppressed) comes from logs or Event Grid.
• Logs: Use ACSEmailStatusUpdateOperational logs for recipient-level details (e.g., DeliveryStatus, SmtpStatusCode, EnhancedSmtpStatusCode, IsHardBounce).

Troubleshooting Steps (Per Microsoft Docs)

1. Verify Domain Configuration:
   • In the Azure portal, go to your Email Communication Services resource → Domains. Confirm "Verified" status for Domain, SPF, DKIM, and DKIM2.
   • Use nslookup to validate TXT (domain verification + SPF) and CNAME (DKIM) records.
   • SPF example: v=spf1 include:spf.protection.outlook.com -all.
2. Enable and Review Logs:
   • Configure Diagnostic Settings for categories: Email Service Send Mail logs, Email Service Delivery Status Update Logs, and Email Service User Engagement Logs.
   • Query in Log Analytics for the Message ID (CorrelationID). Look for recipient-level DeliveryStatus values like Failed, Bounced, FilteredSpam, or Quarantined.
   • Subscribe to Event Grid for real-time events (Delivered, Bounced, etc.).
3. Test Delivery:
   • Use the Try Email feature in the portal with affected Outlook addresses.
   • Monitor the Email Insights dashboard for bounce rates, blocked rates, and engagement metrics.
4. Recipient-Side Considerations:
   • Recipients should check Junk/Spam, Quarantine (Microsoft Defender), Clutter, and use Outlook on the web.
   • Microsoft 365 admins can use Message Trace or transport rules to investigate.
   • Common recipient filters may silently drop or quarantine messages even if ACS reports OutForDelivery.
5. Best Practices for Deliverability:
   • Warm up sending volume gradually (start low and increase over weeks).
   • Maintain low bounce/complaint rates and use suppression lists.
   • Follow sender authentication best practices (SPF, DKIM, DMARC recommended)

Reference:
https://learn.microsoft.com/en-us/azure/communication-services/quickstarts/email/send-email?tabs=windows%2Cconnection-string%2Csend-email-and-get-status-async%2Casync-client&pivots=platform-azportal

https://learn.microsoft.com/en-us/azure/communication-services/quickstarts/email/handle-email-events
https://learn.microsoft.com/en-us/azure/communication-services/concepts/email/email-domain-and-sender-authentication
https://learn.microsoft.com/en-us/azure/communication-services/concepts/analytics/logs/email-logs
Kindly let us know if the above helps or you need further assistance on this issue.

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