Delivery has failed to these recipients or groups

Office 365 users attempts to send an email but receives a bounce back that they are not recognized as a valid sender.  This is often caused by a user sending a mass email flagged as spam.

Delivery has failed to these recipients or groups:

Your message couldn’t be delivered because you weren’t recognized as a valid sender. The most common reason for this is that your email address is suspected of sending spam and it’s no longer allowed to send email. Contact your email admin for assistance.

Diagnostic information for administrators:

Generating server:

Remote Server returned ‘550 5.1.8 Access denied, bad outbound sender. For more information please go to S(9333) []’

Original message headers:

Received: from

([0000::0000:0000:0000:0000]) by

([0000::0000:0000:0000:0000]) with mapi id 000.000.0000.000; Thu, 2 Apr 2020

19:38:16 +0000

MIME-Version: 1.0

Content-Type: text/plain

Date: Thu, 2 Apr 2020 19:38:16 +0000



Unblock a user in Office 365:

*Review that if the users account was compromised and used to send spam that they have changed their password.

  1. As an Office 365 admin, logon to:
  2. Find the user in question, select to Unblock.