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:

• the PC is too slow and/or you have allowed too many concurrent connections. Check the disk fragmentation, also use Task Manager to check the CPU load. If the CPU load is high, and VPOP3 is using most of it, then you may have to decrease the number of concurrent incoming SMTP sessions. Go to Services → SMTP Server → Load Limiting and check the Maximum Incoming Sessions value. Normally this should be about 20-30
• in VPOP3 version 4 or earlier the address whitelist may be too big. This can happen especially if VPOP3 has previously been an open relay. By default, VPOP3 will add all recipients of messages sent through VPOP3 to the whitelist, so if VPOP3 has been an open relay abused by a spammer, the whitelist will contain a LOT of addresses. In these versions of VPOP3, in the VPOP3 directory is a file called 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
• the DNS server which VPOP3 is using may be slow or not responding. Go to Settings → Spam Filter → General and check the DNS Test Results section. Especially check the response time value (especially the max success time value). 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