Error when sending mail: "452 4.3.1 Insufficient system resources"
My solution: make a FULL backup (full server scheduled backup, run at least once) for exchange server. Exchange log will be deleted and free up about 10-20GB disk space. Run backup every month or every quarter depend on number of mailboxes and usage.
Another solution from Exchangepedia is change Back Pressure parameter. Link here: http://exchangepedia.com/2007/03/exchange-server-2007-transport-452-4-3-1-insufficient-system-resources.html