Questions about changing MX record

I have a personal domain where I host email for around 5 family members. I’m thinking about setting up Google’s Postini to do spam filtering, which means I’d change the MX record to send my email to them. But, while I’m pretty savvy about computer technical stuff, I’m weak on my MX record knowledge. Questions:

  1. If I do this, do all of my forwarding rules no longer apply for that domain? Or do they take effect before or after the email gets routed to Postini and back?

  2. Same question for mail filters. I assume these work because they’re just .promailrc files on the final email account repository, but maybe I’m wrong.

And I’m open to suggestions for things other than Postini. I’m getting tired of maintaining my own spamassassin filtering and all the necessary forwarding between accounts that is needed since email was changed to take away procmailrc control. And Dreamhost spam filtering doesn’t cut it. I want something that will:

A) Let me maintain my extensive set of forwarding rules (even if I have to duplicate them elsewhere)
B) Let me keep my same IMAP accounts here on Dreamhost.
C) Do a decent job filtering the spam that ends up in those accounts.
D) Give me an easy way to see what has been filtered out and fix things that were filtered wrong. Easy as in my computer-challenged family members can handle it for their own account.

Hell, I would even consider moving the IMAP accounts elsewhere if it were cheap enough and gave me the forwarding and spam-filtering features. But preferably I’d like my solution to really behave simply like a filter that I dropped in place here at Dreamhost.

What I don’t want to do is move everything away from Dreamhost. I’ve been perfectly happy with them for years, except for these recent email changes. I’ll keep my domains here.