Unable to send email

Stop sending to external recipients and wait until your tenant’s external-recipient count for the rolling 24-hour window falls below its Tenant External Recipient Rate Limit (TERRL); Exchange Online will then automatically allow external sending again.^1^

Check the tenant’s current usage

A Microsoft 365 or Exchange administrator should:

1. Open the Exchange admin center.
2. Go to Reports → Mail flow.
3. Open Tenant Outbound External Recipients.
4. Review the current usage and the TERRL assigned to the tenant.

TERRL is tenant-wide and scales automatically with the number of licenses. A trial tenant is capped at 5,000 external recipients per day. Distribution-list members are counted individually after the list and any nested groups are expanded.^2^

While the limit is exceeded:

• External messages from all users in the tenant are rejected with 550 5.7.233.
• Internal mail is not identified by this error as blocked.
• Retrying repeatedly does not reset the limit; sending resumes only as older recipients leave the rolling 24-hour calculation.^3^

Prevent it from recurring

• Pause scheduled jobs, applications, mail merges, and campaigns that send externally.
• Identify the largest senders using message trace and mail-flow reports.
• Stagger normal external communications rather than sending them in one batch.
• Remember that sending one message to a group containing 1,000 external members counts as 1,000 external recipients.
• For bulk or high-volume external email, move the workload to Azure Communication Services Email or another dedicated bulk-email provider. Exchange Online is not designed for bulk-mailing scenarios.^3^

If the NDR is 550 5.7.236

This is a different limit applying to mail sent from the tenant’s default onmicrosoft.com domain. That domain is limited to 100 external recipients per tenant in a rolling 24-hour period and is intended for testing rather than regular outbound mail.

In that case:

1. Add and configure a custom domain in Microsoft 365.
2. Assign users addresses on that domain.
3. Send external mail only from the custom domain.
4. Wait for the existing onmicrosoft.com recipient count to fall below its rolling-window limit.^1^

If the report shows that usage is already below the applicable limit but the same NDR continues, open a Microsoft 365 support request and include the complete NDR, timestamps, affected senders, and a screenshot or export of the Tenant Outbound External Recipients report.

References

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