> suddenly Gmail was connecting to my mail server over an IPv6 interface Nitpick: when sending email, the sending mail server establishes a connection to the r

> suddenly Gmail was connecting to my mail server over an IPv6 interface Nitpick: when sending email, the sending mail server establishes a connection to the receiving server, not vice versa. The sender is responsible for looking up the domain name's MX record, choosing a hostname from those listed, initiating a connection, establishing an SMTP conversation, and negotiating protocol enhancements like SSL. That said, I could see a change in behavior like this resulting from action on the receiver's side. Though the sending server makes the choice of how to connect, that choice can be influenced by the presentation of the MX record. I haven't specifically run into this before, but I could see this happening if, for example, the MX record advertised a host with an A record first (IPv4), and then was changed to list a host with only an AAAA record first (IPv6). There's quite a lot of arcana in the email space that's difficult to manage without careful, deliberate effort. Sending email is harder than it should be, and a lot of that difficulty results from anti-spam or general anti-abuse efforts. If it was easy to bring a server up and begin sending with a new domain name and IP address while successfully reaching the inbox, then spammers would do exactly that. So new identities are put through the wringer to prove they're legitimate. Well-known platforms have an easier time sending email because they can relay traffic from infrastructure that the ISPs already trust; and in turn they maintain that trust by keeping abuse off their platform.