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Sender's email is rejected and/or marked as spam by Microsoft 365
Published Date : 10 May 2018   Last Updated : 08 Jun 2021   Content Ref: TEC6236806  





Symptoms

You are sending emails from your Microsoft® 365™ email address, but they are not delivered. Instead you receive a non delivery report (NDR) from Microsoft 365, containing the following reason and error code suggesting the source of the issue:
"Remote Server returned '550 5.7.501 Service unavailable. Banned Sender".

Click to view a larger image
  Image showing the error message


Cause

Microsoft 365 has determined that the email patterns, email content, certain words, and\or behaviour, could be an indication that the message is spam. When Microsoft detect what they believe to be spam, they will take action to prevent it. In some cases, this will involve blocking the account sending the mail.


Procedure

  1. Log on to the Microsoft 365 'Exchange admin centre'.
  2. From the left hand pane, click 'protection'.
  3. Click 'action center'.
  4. Wait for the list of account names to load - this may take a little time.
  5. Find your account from the list, select it and click Unblock Account.
Click to view a larger image
  Image showing Unblock Account


More Information

Please see the following Microsoft article for more information: https://docs.microsoft.com/en-us/microsoft-365/security/office-365-security/removing-user-from-restricted-users-portal-after-spam?view=o365-worldwide

You can log a Support request with Microsoft by following the instructions in TEC3877606 in the Other Useful Articles section below.



Other Useful Articles

Microsoft 365 - How to raise a support request with Microsoft (TEC3877606)

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Document Keywords: bounce, reject, rejection, TEC6236806


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