Solving: This message couldn’t be delivered because the sending email address was not recognized as a valid sender

In Microsoft 365, Users are prohibited from sending emails but are still able to receive emails if they go over one of the outbound sending limits outlined in the service limits or the outbound spam policies.

In such cases, when a user attempts to send an email, the message is returned in a non-delivery report with the below error

This message couldn't be delivered because the sending email address was not recognized as a valid sender
This message couldn’t be delivered because the sending email address was not recognized as a valid sender

This message couldn’t be delivered because the sending email address was not recognized as a valid sender. The most common reason for this error is that the email address is, or was, suspected of sending spam. Contact the organization’s email admin for help and give them this error message.

Unblock User from Restricted Entities in Microsoft Defender 365

As an Admin, you can use Microsoft Defender 365 to remove or unblock a user from Restricted Users List by doing the following:

Steps

  1. As a Microsoft 365 Admin, open the Microsoft 365 Defender portal.
  2. At the left menu, and below “Email & collaboration“, click on “Review“, and finally click on “Restricted entities“.
Unblock User from Restricted Users List in Microsoft Defender 365
Restricted Entities in Microsoft Defender 365
  1. Find and pick users you wish to unblock on the Restricted Entities page by clicking on them, then from the above ribbon click on “Unblock“.
Unblock User from Restricted Entities in Microsoft Defender 365
Unblock Users from Restricted Entities in Microsoft Defender 365
  1. Follow the wizard to unblock the user
the user account has been restricted from sending outbound message
the user account has been restricted from sending an outbound message
  1. And it’s recommended to enable the 2FA, and change the user password as well.
Enable 2fa for restricted user in m365 defender
  1. Click Yes, to confirm the unblocking process.
unblock user in Microsoft Defender 365
unblock user in Microsoft Defender 365
  1. Finally, click Submit, and Ok to finish the unblocking request.
confirm unblock user in Microsoft Defender 365

Note: Most of the time, restrictions on the user should be lifted within an hour. Although some technical difficulties could result in a longer wait, the overall wait should not exceed 24 hours.

Conclusion

In conclusion, we have learned why we get this error “This message couldn’t be delivered because the sending email address was not recognized as a valid sender” and how to Unblock User from Restricted Entities in Microsoft Defender 365.