I use Sendgrid <https://sendgrid.com> with an inbound parse webhook which delivers the emails via LMTP to the inboxes and Dovecot <https://dovecot.org> for reading. If you're sending less than 100 emails/day it's free. It's rock-solid and I've been using it for years without any issues at all. I can highly recommend it. Once you've set it up for one domain, adding more is trivial.