O365 and gmail should be honorably mentioned as places that will deliver both inbound and outbound to/from an IPv6-only MX.
A lot other servers will not play ball here. Your self-hosted mail server, if it lacks IPv4, will not get inbound from mailgun, mailjet, github; neither will it be able to send outbound to cisco/iphmx, as well as about 90% of small servers from what I see in my logs.
A lot other servers will not play ball here. Your self-hosted mail server, if it lacks IPv4, will not get inbound from mailgun, mailjet, github; neither will it be able to send outbound to cisco/iphmx, as well as about 90% of small servers from what I see in my logs.