How can I forward incoming messages to a specific MTA based on the username portion of the recipient's email address? I would prefer linux-based software, but Windows would work too.
Our organization is transitioning from ISP-hosted POP3 mail to Microsoft Business Productivity Online Services (Exchange Online) and it would be nice to get coexistence working so that some users could be using the new system while other users continue to use the legacy system. As each user is migrated to the new system, their mail could be redirected to the appropriate server independent of other users and of DNS propagation.
Would this system work? Are there other migration schemes more appropriate to the scenario?
Our current ISP is running Postfix and Dovecot. They provide us with the ability to set up email forwarding, but only to a chosen email address. That doesn't really help us because their server will still use the MX record for delivery, which is what I want to bypass.