I'll try to avoid ranting here, but anything is a legal email address per the RFC (even an @ sign in a username, or an email address without any @ sign).
RFC 821 is the original and 2821 summarizes it plus the few that came after to add and clarify.
The only true "RFC email validity check" is to send an email to whatever address they provide.
Twice now I was only able to register after removing the +[something] part of the email.
Is + actually an invalid email character (according to RFCs etc?). I couldn't find any reference to that when I looked.