550 5.7.236: daily limit for external mail from onmicrosoft.com domains

Every Microsoft 365 tenant gets a default domain ending in onmicrosoft.com, which Microsoft calls the MOERA domain. Microsoft says these domains are intended only for testing, and shouldn't be used for regular email. It enforces a throttling policy: outbound mail from onmicrosoft.com domains is limited to 100 external recipients per organization within a 24-hour rolling window. Once the tenant is throttled, senders get NDRs with 550 5.7.236 for any attempt to send to external recipients.

Two details matter when you count. External recipients are counted after any recipient list expansion, so a group with many external members counts each one. And the limit is per tenant, not per user, so one mailbox sending from the onmicrosoft.com address can use up the allowance for everyone. Inbound email isn't affected.

Microsoft's fix is to buy a custom domain, migrate to it, and make sure only the custom domain is used to send to external recipients. In practice that means adding and verifying your domain, setting it as the default, and changing users' primary addresses, including shared mailboxes and apps that still send as the onmicrosoft.com address. Two related codes cover a different limit: 5.7.232 (trial tenants) and 5.7.233 mean the tenant exceeded its overall daily external recipient rate limit. Microsoft's own figures for trial tenants disagree at the time of writing (November 2026): Microsoft Learn says trial tenants are capped at 5,000 external recipients a day, while the Exchange Team's tenant limits post, updated in August 2026, says 500. Plan around the lower figure.