As PlusNet's spam filtering is not perfect, I have been looking for something to help do the job. I have discovered a little tool that runs on Unix/Linux called imapfilter. It is not pointy-clicky but is very powerful. I run it periodically from cron and am using it for the following:
* Deleting messages with a To: field set to my old deleted addresses.
* Deleting messages sent to email@example.com as I only use my domain addresses.
* Deleting the "Alert: virus etc..." messages.
* Moving any messages that Thunderbird marks as junk to a central location for checking and forwarding to despamchecker (it could do this automatically but I prefer to check them first).
* Deleting anything from my Spam folders with certain keywords or phrases in the subject (I am quite prepared to accept that I and my users do not want to receive any mails about Viagra or Wonder*** etc.). This makes manually checking for false positives easier.
* Deleting anything from some inboxes with a different list of keywords in the subject.
* Checking how many mails all my users have in their Spam folders.
I expect I will think of lots more way to use this. All you need is access to a Unix / Linux box, it doesn't have to be the one where you read mail. It is irrelevant what mail client you use, imapfilter is standalone.