It should be smooth as silk. You can have both POP3 polling and direct delivery active at once, so the sequence would be something like:
- Setup direct delivery.
- Change the forwarding rules (or MX config, or whatever) to deliver to the direct delivery MTA rather than the POP3 mailbox.
- After a while (once you’re pretty sure no more mail’s going to land in the POP3 mailbox, due to cached MX records or whatever), nuke the POP3 polling config.
You’ve just got to do something to map incoming envelope-to addresses to site hostnames, and then modify mail-receiver to set the Host: header on connection (if it’s a multisite container), or connect to a different host/port (if separate containers). Since that is exactly how we do it here at Discourse (we lookup the envelope-to address in our master hosting database, and then connect to the appropriate site),
mail-receiver is structured to make that pretty easy – just build a new container with an alternate entrypoint script that includes the default script then overrides
post_email. If you need assistance, I’m happy to give specific pointers and share bits and pieces of how we do things privately.