…which both excludes addresses allowed by the RFC and includes addresses disallowed by the RFC. (For example, the RFC disallows two consecutive dots in the local-part.)
I take the descriptivist approach to email validation, rather than the prescriptivist.
I know an email has to have a domain name after the @ so I know where to send it.
I also know it has to have something before the @ so the domain’s email server knows how to handle it.
But do I care if the email server is supports sub addresses, characters outside of the commonly supported range (eg quotation marks and spaces), or even characters which aren’t part of the RFC? I do not.
If the user gives me that email, I’ll trust them. Worst case they won’t receive the verification email and will need to double check it. But it’s a lot better than those websites who try to tell me my email is invalid because their regex is too picky.
The HTML email regex validation [1] is probably the best rule to use for validating an email address in most user applications. It prohibits IP address domain literals (which the emailcore people have basically said is of limited utility [2]), and quoted strings in the localpart. Its biggest fault is allowing multiple dots to appear next to each other, which is a lot of faff to put in a regex when you already have to individually spell out every special character in atext.
I generally agree, but the two consecutive dots (or leading/trailing dots) are an example that would very likely be a typo and that you wouldn’t particularly want to send. Similar for unbalanced quotes, angle brackets, and other grammar elements.
That would be bad form, IMO. The user may have typed john..kennedy@example.com by mistake instead of john.f.kennedy@example.com, and now you’ll be sending their email to john.kennedy@example.com. Similar for leading or trailing dots. You can’t just decide what a user probably meant, when they type in something invalid.
Yeah, that's about as far as I've ever been comfortable going in terms of validating email addresses too: some stuff followed by "@" followed by more stuff.
Though I guess adding a check for invalid dot patterns might be worthwhile.
