If you get this error appearing when messages are sent to VPOP3, either in a bounce message, or in the VPOP3 status monitor, it means that the VPOP3 computer filter is taking more than the allowed amount of time (usually 60 seconds).
The allowed time is set in Settings → Spam Filter → General → Enable Spam/Content filter on SMTP messages - timeout XX secs
In normal situations the content filter should only take 2 or 3 seconds at most, so if it is taking more than 60 seconds, there is usually a problem.
Some possible reasons for it taking too long:
Maximum Incoming Sessionsvalue. Normally this should be about 20-30
WHITELIST.TXT. If that file is over 5MB or so, then this may be the problem. Stop VPOP3, rename/move the WHITELIST.TXT file, and restart VPOP3 and see if that makes any difference. In VPOP3 v5 and later, the whitelist is stored in the database, so does not affect performance as much if it is large
max success timevalue). If these are showing an error or are over a few hundred milliseconds, you may have an issue with the DNS server which VPOP3 is using