New article in the Exchange 2010 MP online library recently.  Read more about it here: http://technet.microsoft.com/en-us/library/ff360301(EXCHG.140).aspx

Taken from the article:

Explanation

This Error event indicates that end-to-end e-mail message flow is taking longer than 90 seconds for more than percent of all messages that have been handled over the previous 30 minutes.

This event may be triggered when one or more of the following conditions are true:

  • A networking issue exists.
  • The Microsoft Exchange 2010 Transport server is under a heavy load.
  • The particular messages have been sent to large distribution groups.
  • A Transport Agent issue exists. For example, the antivirus agent or Rights Management Services agent may be experiencing an issue.

User Action

To resolve this problem, do the following:

  1. Use the Message Tracking tool in the Toolbox in the Exchange Management Console to determine which messages have taken more than 90 seconds to be transferred. For more information, see Understanding Message Tracking.
  2. Examine each message to determine the reason for the slow delivery time.
  3. Look for configuration errors. For help to determine what might be configured incorrectly, run the Microsoft Exchange Best Practices Analyzer, which is located in the Toolbox. For help to troubleshoot mail flow issues, run the Mail Flow Troubleshooter, which is located in the Toolbox.

If you are not already doing so, consider running the Exchange tools, which have been created to help you analyze and troubleshoot your Exchange environment.