"550 5.7.1 Service unavailable" nondelivery report (NDR) error code in Forefront Online Protection for Exchange

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Article ID: 2666743 - View products that this article applies to.
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Email traffic is rejected when the traffic is sent to a mail environment that uses Microsoft Forefront Online Protection for Exchange (FOPE), and this includes a Microsoft Office 365 environment. A nondelivery report (NDR) is generated that contains the following error code:
550 5.7.1 Service unavailable


The email server that is sending the email messages is listed in the FOPE Blocked Sender list. The email server is listed because of previous email activity or concern from the sender's email address.


First, investigate and resolve any malicious activity, malware, or virus issues that could cause spam activity from the email address that is sending the email messages. Then, contact FOPE Support to remove the sender's email address from the Blocked Sender list. This procedure is known as "delisting."


Create an exception policy rule to let sender traffic that is blocked into your mail environment through FOPE. For more information about how to do this, visit the following Microsoft websites:


Article ID: 2666743 - Last Review: June 14, 2013 - Revision: 7.0
Applies to
  • Microsoft Forefront Online Protection for Exchange
  • Microsoft Office 365 for education  (pre-upgrade)
  • Microsoft Office 365 for enterprises (pre-upgrade)
  • Microsoft Office 365 for small businesses  (pre-upgrade)
vkbportal225 vkbportal231 vkbportal237 o365 KB2666743

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