Do you notice if it happens at a particular time everyday?

Cause we’ve discovered that it can be a cron problem. Try disabling all cron jobs and see if it still happens.

Also, be sure to do this:

Go into plesk:

1) Mail server settings

* At Relay Options:
2) Uncheck the authentication required: “POP3 lock time”

* Disable greylisting:
3) /usr/local/psa/bin/grey_listing –update-server -status off

http://forum.parallels.com/showthread.php?t=90550