> (The reason to do this is to make the move over seamless for the user of that > domain, and that it how their previous host had the mail setup. Duplicating > the setup means I do not have to go in and change the mail servers on every > client computer, phone, and tablet, so this is desirable. But secure > submission is required by the server at all times, so if that gets messed up, > I've gained nothing.
This seems a little confusing and maybe convoluted to me, unless im reading this wrong. MX records and settings in user's mail client programs are two different things. A user sending an email from a client program like Thunderbird does not use MX records. It looks up the IP and port of the domain saved in the "server name" and "port" settings. You can change an MX record and not need to tell the world to update anything, that is how MX works. If someone wants to send an email to u...@otherdomain.com the delivering smtp server (like postfix) looks up the MX record for otherdomain.com, sees that it is mail.example.com then looks up the IP for mail.example.com. None of this process has anything to do with end users or their configuration. The only thing you need to worry about with end user setups, if the goal is keeping the same domain/hostname they are using in their settings, is to modify the existing DNS A record (like imap.otherdomain.com) to point to the correct (whether same or different) IP they need to connect to for submission. Completely different process than MX records.
