[Hampshire] Linux SPAM filtering

Can anyone on this Linux group recommend a SPAM filter solution on Linux for
multiple domains - we want to host a virtual Linux server - stream email
from 10 domains through it, then pass the filtered email back to our
seperate domains for further domain specific (keyword for example) cleaning
before hiting the mail server for distribution - the servers may be MS
Exchange or may be Send Mail - also one domain sends to Windows Mobile -
but this is a POP system so no big deal - 3 of our domains receive up to
15,000 spams a day and the server overhead is a bit stupid so we want to
pass this overhead over to a hosted solution on a virtual server - thanks
for your advice in advance - Richard