Preferably also PostgreSQL, but currently I get answers like "No reliable IPv6 email sending to the big names" and "IPv6 not supported for receiving".

The reason that I'm on this discourse, is that I'm busy deploying IPv6 only hosts, and Mailgun for one doesn't have an IPv6 SMTP only IPv4 ;( and the others I've found either way too expensive or also not IPv6 capable, so I started looking at roll my own... and ran into these constraints just as I'm ready to swallow the docker only options for systems like Postal.io and Hyvor Relay