Do you run your own email server?
If your ISP does this sort of thing, then it's up to them.
If you do have enough control to add extra things, then as a basic solution, try SpamAssassin with procmail.
A very good solution would be to use IronPorts, but this costs much $$$ (my email hosts use these).
Suggested solution:
- Get a gmail account (great spam filtering)
- Set up serverside to forward all your mail to your gmail address
- Set gmail to send all mail in its inbox to either another email address of yours (if you have your own domain, for example), or just get your gmail with Outlook (both POP3 and IMAP supported). Ten set Gmail to archive all sent mail.
Spam gets filtered through Gmail, so check your spam folder in gmail periodically for false positives. As a bonus, you get an automatic backup of all your mail.