spamassassin is what i use mainly, there was an outlook plugin for it, but i think its gone quiet, so of the non/commercial plugins are based on spamassasin too.
symantec , high quality, same sentence?
i find most anti spam systems to be like most anti virus products, only good at catching the same stuff as everyone else, and missing all the new stuff, its either rules based or heuristic thats easy to break, whitelisting is getting to be the only good way i'm finding, but its got that problem with the first email.
i used to dislike that system where you send someone an email and it sends you back a confirmation catchpa, i know people get a bit self indignant about getting them too, but they're seeming like a better idea now.
I loose so many valid emails because of the ISP filtering or bad heuristics and word tags, especially since a lot of people tend to write short notes with a url.