With 7 (active) email addy's I've been tending to get a decent amount of spam, Thunderbird would catch about 90% of it though that ment 2-3 would still get through a day this was primarily all from my pop3 account with the ISP and Uni account. My webmail accounts (gmail, yahoo, hotmail) with there own built in filters would catch about 99% of the spam that went there way, and any that got through them seemed to have a 50:50 chance of getting through thunderbirds filter.
In an effort to cut down the spam I looked at setting up spamassassin, unfortunately tough it became apparent that thunderbird while a reasonable email client in most cases has absultely no support for working directly with spam assassin.
As a result instead I decided that my main gmail account would also collect my maxnet email which I'd then retrive from gmails pop acceess, so a which visit to gmails setting and it's done.
Over the next couple of days I noticed a significant decrease in spam happily enough. Now checking my gmail account I note that the spam folder had gone from an average 70ish spam to 130, a quick look showed that I was averaging 2 spam a day to my gmail account and 25+ a day to my isp address.
So I'd suggest if your getting alot of spam through an email account thats not gmail see if you can set it up so gmail checks it for you, seeing there spam filter is rather good!
Moral of the story is:
Gmails spam filter rocks!
Gmails fetch other mail is cool!
Thunderbirds is meh!
Thunderbird should directly support spamassassin!
So if you don't have a gmail account get one and start using it, so instead of worrying about spam you can worry about gmail scanning your email! :)