you have the main problem in hand. You’ll still need to do all the DKIM / rDNS stuff to be certain your mail is accepted, but using SES as the source gives you a significant leg up vs originating locally. I don’t see why you can’t run dovecot and postfix on separate systems, but a single VM isn’t bad if it’s properly secured. Hosting SMTP/IMAP is not that difficult but you need to make sure you don’t accidentally misconfigure things and become an open relay - as with all internet facing systems, mail services are targeted constantly so you should use fail2ban to deter them.