Good evening!

We are trying to set up zimbra to use exchange instead postifix to send internal and external mails ( it's not the definitive solution, we need it for migration purpose).

Actually the server ( ) is configured this way:

Internal MTA : local postfix ( )
Relay host : ( is uses authentication , we used this tutorial )
Everything is working great, internal mail ok, external too.
The problem is that we have also an exchange server, we are trying to kill it ( XD ). To keep internal and external mail sincronized we'd like to send EVERYTHING via exchange. Just for a while, when the migration is ok we will return to the default config.

The target configuration:

Internal MTA: exchange ( )
PROBLEM: exchange needs authentication. How can i provide auth to exchange when sending mails?
I'm getting mad to find a link or a tutorial to do this.

EDIT: Forgive my ugly english, i'm italian