I tried running an email server a couple times, it is pretty intense
I just run my own, but I warn you .. it's not for the faint of heart. My email is literally the most complex service I self host. It's probably wise to just pay for a good provider:
https://battlepenguin.com/tech/upgrading-opensmtpd-6.3-and-running-e-mail-in-docker/
Discussion
DKIM is black magic to set up properly