@m
How is it 'effectively impossible'? I don't see that, and have been running my own mail server for almost 30 years now...

@m
Several times. Yes, that is not something you do overnight. My procedure there is, set up the new server as a secondary MX (but don't listen on it, yet) and update the SPF record to also mark it as a valid source, then wait a month for the reputation to catch up before switching over.

I need that much time to switch everything else over, anyway.