If the VPOP3 spam filter appears not to be working at all, the first thing to do is to look at the message headers of an incoming message to see if the spam filter is being run at all.
Check to see if a header line beginning with X-VPOP3-Spam is in the message headers.
If the X-VPOP3-Spam header is not in the header, then the spam filter is not processing the message.
Some possible reasons for this are:
If the X-VPOP3-Spam header line does exist in your incoming messages, then the VPOP3 spam filter is processing the messages, so there must be something making the spam filter think that the messages are not spam.
The most common reason is that someone has added the sender or common text into a spam filter whitelist (Settings → Spam filter → White/Black lists). This may be via a wildcard, so may not be easily seen. If this is the case, then the X-VPOP3-Spam header line will contain a reference to 'whitelist'. The 'X-VPOP3-WL' message header may contain more information.