Half OT; smarthosting postfix whilst migrating from exchange 5.5

I have an itch that I can't scratch. And I am not sure how to approach the
problem.

I am in the process of moving from from windows servers to linux ones.
Samba, Postfix, cyrus and OpenExchange.

I am following the recommendations in Sun's migration whitepaper to slowly
move from exchange to postfix a few users at a time.
http://www.sun.com/servers/entry/v60....migrating.pdf

To that end this doc is great:

http://www.unixwiz.net/techtips/post...nge-users.html

I will get the valid exchange users email addresses onto the postfix server
and then relay certain users to the exchange server.

But as I move users over to the postfix server I wan't postfix to deliver
locally to those users that have been migrated. I gather that
http://www.postfix.org/STANDARD_CONF...ON_README.html

for a combination of "Postfix on a local network" & "Postfix email
firewall/gateway" and "Delivering some but not all accounts locally" would
be most appropriate to what I am doing. But I am not 100% sure how to
approach this. Does anyone have a recipe that I can learn from?


The other problem which is OT, but I am hoping someone can give me a hand
with...... Is that users on the exchange 5.5 server will still need to
email users on the postfix server. I currently operate a flat domain
structure so again am unsure of how to tackle this. I've googled a lot for
info on how to get email routed from jill@example.com (exchange server) to
jack@example.com (postfix server) and don't know how to approach this... I
can't find any global setting which would mean that a user not in the global
address list gets forwarded to the postfix server even if the exchange
server think's it owns example.com and the user is eg jack@example.com (on
postfix).

Or would it be easier to delete the users mailbox on the mailserver, add
them as contact in the global address list and point their smtp at
jack@postfix.example.com and have an MX record for that server in local DNS?


Kind of as per this knowledgebase article? :
http://support.microsoft.com/default...b;en-us;217217

Any suggestions on how to approach this for a shakey postfix beginner?

Regards Geoff