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

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:

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:

• Create, Edit, or Delete a Policy Rule
• Understanding Policy Rule Match Options