I use Roundcube for webmail, which auto-collects outgoing addresses in an address book (and you can obviously manually add addresses as well). I have a script t

I use Roundcube for webmail, which auto-collects outgoing addresses in an address book (and you can obviously manually add addresses as well). I have a script that periodically exports the address book for each user and uses that as a post-Bayes filter; anything from a known address is automatically whitelisted, and if it would have been marked as spam by Bayes then it is also trained as ham. It works pretty well.