How to replace the mail server?

Question

postfix

Given:
Old Postfix-based
mail server New Exchange-based mail server
Required:
Gradually, slowly, one by one, migrate all users from Postfix to Exchange.
One nuance - the external domain is the same everywhere.
What I've done.
Set up Postfix on Exchange as a smarthost. Mail with Exchange outside successfully goes.
What thoughts.
I think we need to do mail forwarding from Postfix to Exchange. But the problem is that you need to send not everything at once, but by family name. That is, first one dozen users, then another, and so on. So that admins have time to reconfigure mail programs for users.
Tried through aliases - did not work. It is necessary to raise one more domain, even a fictitious one. And do not want to.
Do you have any ideas or ready made solutions?
PS.
Roughly speaking, it is necessary that a letter that comes to Postfix for [email protected] falls into a local mailbox, and for [email protected] it is redirected to the Exchange server

Answer 1

In general, I asked myself: I myself answer.
On Exchange - we start an external domain, the same as on Postfix. As a result, there are two domains. One is external, the second is internal, which is installed when installing the program. In our case, these are: domain.ru and ad.domain.local
domain.ru - we make the default domain
On Postfix, in aliases we forward for [email protected] to -> [email protected]
Accordingly, this vasya should be on both servers.
As a result, the mail arrives in its normal form, without the recipient's address being changed. Since both servers understand domain.ru
Reception is configured.
And I just sent it via Postfix. On Exchange configured it as smarthost.
there will be questions - write.
PS. There is really one nuance. At the time of the transfer, you cannot send mail to an employee who has not yet been transferred. The idea is to make an alias, already on the exchange. I haven't thought about how to do it better yet. How to do - unsubscribe