I wouldn't go so far as to call it an antifeature--in fact, I wouldn't be surprised if a common use is to allow people to maintain multiple accounts on the same

I wouldn't go so far as to call it an antifeature--in fact, I wouldn't be surprised if a common use is to allow people to maintain multiple accounts on the same service with one email. It isn't standard, but it's not in violation of any standard--nothing says that the server must store each distinct valid email in a separate mailbox with its own login. Many servers implement "catchall" emails or aliases which result in the same thing, distinct addresses going to the same mailbox. What is a problem, and what is non-standards-compliant, is GitHub incorrectly assuming that all mail providers will do this when many do not. It would be no different from assuming that "admin" and "postmaster" go the same mailbox because that's the way a lot of software is configured.