I have someone with two domains, a.com and b.com.

a.com is running a postfix server on the mx host for the domain and I have total control of the server.

b.com is running an exchange server on the mx host for the domain and I do not have any control of this server.

They have been using b.com as their primary mail address and use the exchange calender with outlook.

They want all the same functionality but want to start using a.com as primary mail address.

I opened up postfix to allow relay from the ip address of the exchange server and hopefully that's enough from the outgoing side.

For delivery though what can I do to forward all incoming emails to the exchange server? I have some aliases defined in /etc/aliases that should take higher priority.