Below you will find a link to the Microsoft explanation, and to some other work around for this issue.

452 4.3.1 Insufficient system resources - on relay of: mail from

A reputable resolution can be found at the following URL:

http://exchangepedia.com/blog/2007/03/exchange-server-2007-transport-452-431.html

The Resolutions:

1. Disable Back Pressure (https://exchangepedia.com/2007/03/exchange-server-2007-how-to-turn-off-the-back-pressure-feature-on-transport-servers.html) or
2. Modify the parameters to more accurately define what’s high utilization for your deployment or server configurations or
3. Move the Queue database to another volume with ample of free space (http://technet.microsoft.com/en-us/library/bb125177.aspx)

If you have gone through the above suggestions with no luck. Click the link below for Microsoft's explination, and fix.

http://technet.microsoft.com/en-us/library/bb201658.aspx