Shawn Hogan from DigitalPoint actually started filtering his various corporate email accounts through the gmail mail server. Shawn used many types of spam detection and filtering systems, including Spam Assassin, which I am happy with, to block unwanted email. But for some reason 2,000 spam emails seemed to slip through his filters on a daily basis. Shawn decided to to use gmail to filter his email address and it worked.
How did he do it?
Forward all email to gmail, access the gmail email via POP in his client side email program, and send email from his corporate email addresses like he normally would. He doesn't touch gmail.