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.
A lot of valid email addresses are obvious typos. steve@gmail is perfectly valid but useless in most web forms, for instance. A lot of websites drop technical compliance for the convenience of people who don't know how email works.
Technical compliance can also become rather annoying when you start doing things like escaping characters in quoted strings or include spaces. Practically nobody is using any of that stuff in the real life, so you rarely ever need full compliance.
I don't know why single character email addresses would fail that test, though.
I don’t know why single character email addresses would fail that test, though.
Could be that they get a huge amounts of bounces from those kinds of addresses. I'm sure at least half of Germany is using a@bc.de as the go-to "I don't wanna give out my email" address.