Ho to Solve returning external emails

The 550 5.7.233 NDR means the tenant has reached its Tenant External Recipient Rate Limit (TERRL), so external sending will resume automatically when the recipient count for the rolling 24-hour window falls below the tenant’s limit.^1^

The Message-ID ending in PROD.OUTLOOK.COM is only the message identifier; it is not the rejection reason.

Restore external mail flow

1. Pause nonessential external sending across the tenant.
   TERRL applies to the entire organization, so moving the send to another mailbox does not bypass it. Distribution-group members are counted individually after expansion.^2^
2. Wait for the rolling 24-hour count to decrease.
   This is not necessarily a fixed midnight reset. Each recipient drops from the count 24 hours after the relevant message was sent. Once usage is below the limit, users can send externally again.^1^
3. Check current consumption and the tenant limit.
   In the Exchange admin center, go to: Reports → Mail flow → Tenant Outbound External Recipients.
   This report shows the tenant’s TERRL and usage.^3^
4. After usage falls below the limit, send one external test message.
   Confirm that it is delivered rather than generating another 5.7.233 NDR.

TERRL scales automatically with the number of licenses in the organization. It is not an Outlook Classic setting, so reinstalling Outlook, recreating profiles, or changing client settings will not resolve this tenant-wide block.^2^

Identify what consumed the allowance

If the volume was unexpected:

• Use Exchange admin center → Mail flow → Message trace to identify the mailboxes or applications responsible.
• In the Microsoft Defender portal, review the built-in alerts:
  • Email sending limit exceeded
  • Suspicious email sending patterns detected
  • User restricted from sending email
• Investigate any unexpected sender activity as a possible compromised account before restoring normal sending.

These monitoring and alert options are documented in Microsoft’s outbound-limit troubleshooting guidance.^2^

Prevent recurrence

• Spread legitimate external sends over time.
• Remember that a group containing 1,000 external members consumes 1,000 external-recipient units.
• Create a custom alert at a suitable threshold—for example, 80% of TERRL—so administrators can reduce or redirect sending before the tenant is blocked.
• For bulk, marketing, billing, or other high-volume external mail, use Azure Communication Services Email or a dedicated bulk-email provider. Exchange Online is not designed as a bulk-mail platform.^2^

If the EAC report shows usage below the displayed TERRL but new messages still return 550 5.7.233, open a Microsoft 365 support request and include the full NDR, UTC timestamp, sender, recipient domain, and a screenshot/export of the Tenant Outbound External Recipients report.

References

1. Email nondelivery reports and SMTP errors in Exchange Online
2. Troubleshoot outbound sending limits and blocked users in Exchange Online
3. Exchange Online limits