Non-delivery report 550 5.7.1 Unable to relay this email
Problem
When you send an email, the email is not delivered, and you receive a non-delivery report with this error message:
550 5.7.1 Unable to relay this email
Reason
This error will be returned if you have not removed the server-side mailflow rules and connectors after your trial has ended, or you have moved away from the service and disabled your tenant.
Once configuration details for your Microsoft 365 tenant are no longer available, Signature 365 is unable to route email back to your Exchange Online instance when it is routed to the Signature 365 systems.
Solution
To resolve this issue remove the mailflow rules and connectors sending email to Signature 365 by following the article below:
Non-delivery report 550 5.4.1 Recipient address rejected: Access denied
Resolve 550 5.4.1 recipient address rejected errors, especially for public folders routed through Signature 365.
Non-delivery report 552 5.3.4 Header size exceeds fixed maximum size
Troubleshoot 552 5.3.4 header size errors when oversized headers block Signature 365 mail processing.