This is a really interesting concept.

Basic Procedure

Gmail allows you to forward incoming messages to any other e-mail address. Go to the Settings page and then to the Forwarding and POP tab. In the Forwarding option, set Gmail to forward all incoming mail to your regular e-mail account, and keep a copy in Gmail’s inbox.

(In this explanation, I will assume your regular e-mail address is user@domain.com and your Gmail address is user@gmail.com, and I apologize to Mr. User over at Domain.com and Mr. User over at Gmail if they get any extra mail from people following the steps in this article too literally.)

Once that forwarding rule is set on Gmail, all incoming mail to user@gmail.com will get spam filtered and anything left over will be forwarded to user@domain.com, with a copy left at Gmail. But that doesn’t help you much yet, because people are still sending spam directly to your user@domain.com account.

Now, over at your user@domain.com’s mail server, create a server-side filter to check the headers of any incoming e-mail. Have it forward to your Gmail account if it does not find the following in the header:

X-Forwarded-For: user@gmail.com user@domain.com

In English, the filter would be written: “Any mail that does not contain ‘X-Forwarded-For: user@gmail.com user@domain.com’ in the mail header should be forwarded to user@gmail.com”.

Once this server-side filter is in place, only mail on its way back from user@gmail.com (already filtered for spam) will be passed to your user@domain.com account’s inbox. Everything else will be forwarded on to user@gmail.com to be filtered and forwarded back.