custom MAIL server -> Proxy Server -> Gmail Server

So I have some custom VPS which route emails via MX record in DNS. And I need to setup gmail interface via Google Apps - this step and previous are clear. But how can I insert some middle layer, to check emails messages for special words/etc., so something like spam assasin proxy, but custom product.

The question is: How could i setup proxy mail from my server => to proxy server(or application) => to gmail servers?

Thank you, for any help!

From serverfault Eugene

• Mailscanner can do the checks that you want to perform.

  Eugene : thanks for response, but I'll need to check each email for *my custom* filters. and I didn't get it, how to setup it before gmail? because gmail is work via mx-record, so its mean that installed mailscanner or whatever else will not got any mail, isn't it? so how do I hack this stuff? thanks.

  topdog : You cannot do that if you want to do your own checks then you have to get the mx records to point to you. Then you deliver to gmail

  Eugene : but how do i transfer mail next to gmail server after my proxy to gmail server

  topdog : Depends on your proxy, you just need to tell it the final destination is the google server.

  Eugene : can you give me a sample of such configuration or proxy product url, please?

  Eugene : sir, please advice a product for this goal?
  From topdog