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