Any Postfix solution for mail server migration?

[Swift Griggs <swiftgriggs%gmail.com@localhost>]

How can this be done? I've Googled and there only seem to be vague hints, 
but no solid method to migrate in the way I'm proposing.

---[ Problem Description ]---
Server "Oldserv" = old MTA system I want to get away from
Server "Newserv" = new MTA I want to migrate TO.

1. Mail to flow to Newserv first.
2. If mail is for a local user, the mail stays on the system.
3. If the mail is for an unknown user it's forwarded to Oldserv.
-----------------------------

I don't want the header getting re-written. So, I see people doing things 
like setting up aliases so that userpart%mydomain.com@localhost gets changed to 
userpart%oldserv.mydomain.com@localhost. That's unacceptable. It'll hork up the 
user's mail experience and re-write addresses that were fine the way they 
are.

The whole point of this is creating a method where I can migrate one user 
at a time to the new system without having to do a "big bang" migration of 
the whole company at once. The logistics of that are too big for me. So, I 
have to be able to take it on in chunks.

Plus, I'm not married to Postfix. I'd use Exim or anything else if it'd 
work. The postfix mailing list and IRC channel are super-hostile & 
abusive. I certainly wouldn't be heartbroken to never have to communicate 
with them. This is one of many reasons why I run NetBSD rather than 
OpenBSD ... Community attitude/spirit also matters to me.

This is a simple "checkbox" feature in Communigate Pro, but I'd rather 
stick with open source, given a choice.

 Swift