Outgoing message delivery is delayed or rejected in Forefront Online Protection for Exchange and you receive a "Greylisting in action, 451 4.7.1" nondelivery report

In a Microsoft Office 365 pre-upgrade environment, your outgoing (also known as "outbound") email message delivery is delayed or rejected in Microsoft Forefront Online Protection for Exchange (FOPE). Additionally, you receive the following nondelivery report (NDR) error message:

This issue can occur if the receiving domain enforces a retry period that's designed to block spam. This causes message delivery to fail because greylisting blocks all senders until they pass the greylisting test at least one time. Greylisting systems frequently encounter issues when they retry from different Message Transfer Agents (MTAs).

To resolve this issue, follow these steps:

1. Troubleshoot issues that are related to Simple Mail Transfer Protocol (SMTP) and mail flow. To do this, go to the following Microsoft TechNet website:
   How to Use Telnet to Test SMTP Communication
2. If SMTP communication works, contact the recipient administrator directly to request that they add your sending IP address to a safe senders list. These are listed in the FOPE Administration Center.

For more information about greylisting, go to the following website:
The receiving mail server tries to validate a sender by using a triplet set of information. To form a triplet, this server checks the following three pieces of information about any particular mail delivery attempt:

• The IP address of the delivery host
• The envelope sender address
• The envelope recipient address

For security, the triplet must have the same three pieces of information for two consecutive delivery attempts. The first delivery is always rejected as greylisted or unsafe. However, because data centers send from many MTAs, the greylisting encounters issues in retry mode when it tries to build a triplet to validate the sender.

Microsoft provides third-party contact information to help you find technical support. This contact information may change without notice. Microsoft does not guarantee the accuracy of this third-party contact information.

Still need help? Go to the Office 365 Community website.