Huh? RFC 2822 describes in detail what a valid email address looks like. (There might be newer ones superseding it; that's the one that was current back when I looked into this topic.)

In practice, a lot of theoretically valid email addresses are rejected by implementations, e.g. my favorite:

  "@"@example.com

What's true, of course, is that you should never rely on an email address being syntactically correct. You should only use it after having done a validation process that involves sending mail to that address.