If you get this message in VPOP3 it means that VPOP3 is trying to forward a message on to another mail server (probably Microsoft Exchange Server), and that other server is rejecting the message with the error 452 4.3.1 Insufficient system resources

You will need to fix the problem on the Exchange Server, it may be due to lack of disk space.

One of our users reported that they ran CLEANMGR on one of the partitions of the Exchange Server and freed up some disk space to resolve the problem.

If this doesn't help, you may need to contact Microsoft's technical support for help with Exchange Server.