Apologies for not yet posting my reply on the big spam issue -- hopefully later today
However, on the issue of Spamassassin, it 's a very good piece of software, however, it does require quite a lot of "grunt" if you recieive a lot of email. Does your hosting provider indicate how it's integrated into the mail delivery process on your system ? There are a number of ways of doing this, which have their own pros and cons.
HTH,
RTFM