I've used Y!Mail in the past, and been pleased overall. However, while their filters do catch about 95% of the Spam, I had problems with false positives. I, personally, would rather see a bit more pink meat in my mailbox and be assured that false positives would be very rare.
I've been pleased with GMail in this regard, but for real "industrial" use, I use Spam Assassin with a whitelist and a threshold score of 6.5. That seems to nab about 80-85% of the Spam, and I have had 0 false positives in about 6 months.
Anima Legato .oO all things connect through the motion of the mind
The only false positives I've experienced was when I accidentally
marked one "mod_perl digest" as SPAM.
It took about 5 trips to the SPAM folder to click "NOT SPAM" to start recieving the digest in my inbox again.
The onl things that slip past are
bounce-backs, very annoying and the filter seems to filter them like other spam, instead of checking that the original sender is *.mail.yahoo.com, not some russian website.
new nigerian 419 variations, but those are sooo easy to pick-off
MJDsays "you can't just make shit up and expect the computer to know what you mean, retardo!"