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.
As we no longer have configuration details for your Microsoft 365 tenant, we are unable to route email back to your Exchange Online instance when it is routed to our systems.
Solution
To resolve this issue remove the mailflow rules and connectors sending email to Signature 365 by following the article below:
Removing the Signature 365 Mailflow rule and connectors