mailbox@[x.x.x.x] for IPv4, mailbox@[ipv6:...] for IPv6...
And see how he will deal with the Big Tech email services blockade... They should get huge fines as they are forcing all email services to go into useless complexity, and sometimes a complexity you have to implement AND pay for access.
It is not anymore "You've got mail", it is "you are blocked" (even though, there is a strong SPF like spam scoring indicator with with the email address being contained directly in the actual IP of the sender/receiver...).
gmail is not email anymore, it is Big Tech messaging, compuserve and AOL all over again. They are injecting their poison in people slowly but surely, but they hav e the means to be very patient with their billions of $... ohoho, you'll get to use their messaging platform or existing will become a pain.