As a person who ages ago created and single letter (before the @) email address thinking myself clever and efficient... I'm amazed and distressed how many forms have insisted that my email address is invalid.
Hmm. Why am I mildly surprised that I can't find anything non-regular about the syntax. There's nested comments but that's part of MIME quoting, not the actual address format, so it's reasonable to not accept those in an HTML entry field because HTML is many things, but not MIME.