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