This is all a massive misunderstanding. An email address is the local name and the host; a host can't contain an @, so the only thing you frankly need do is split on "last @" and demand the user not escape anything. As for validation, go ahead and try to resolve the domain to make sure it works (and, if you want to verify the local part, do an online check with their server).

If this squicks you for some reason--as maybe that format is non-obvious with respect to the lack of a need to escape @--give the user two boxes with a hardcoded @ between them and have them type the two parts separately: pre-parsed input need not ever be escaped, as you aren't going to parse it at all; no need to implement " dequoting.

All of these escaping rules are then to support embedding this identifier into SMTP. The rules for embedding the same identifier into MIME are different... and even more complex! In MIME they support random stuff like "comments" in the middle of the string... is that part of the email address identifier? No.

An email address simply is not defined by the format you use to send it as part of an SMTP command, nor is it defined by the format you use to send it as part of a MIME message header :/. Into is an identifier that exists separately from either of those two (different) protocols and one would expect any number of ways to escape that content.

To demonstrate how ridiculous this all is, imagine someone comes up with a JSON protocol for mail submission and then documents how email addresses now should use \u encoding and escape quotation marks... does that mean users should type that into your app? No.

Hell: your email address form is taking an email address and then sending it over HTTP... the escaping rules for HTML form fields are different still, yet no one is asking users to type HTML-escaped strings into other applications, right?

The core thing wrong then with your email validation is that you are simply validating the wrong thing: unless you are developing an SMTP server, the rules for how to escape and parse escaped email addresses in RFC5321 are irrelevant; and, likewise, unless you are developing a MIME parser, the rules for how to escape and parse escaped email addresses in RFC5322 are also irrelevant.

The only thing that matters from either of these specifications is the underlying basic rule for what semantically can exist in a hostname and a localpart, and RFC5321 is extremely lax: you can use any "ASCII graphic or space", and so excludes only ASCII control characters and 8-bit characters... and then, as mentioned, another RFC removes the 7-bit limitation and opens up the world of Unicode.

(To push on it even further: it isn't even clear to me that one should consider the ASCII control character limitation to be fundamental to the email address identifier or a weird limitation of the current version of SMTP; and since none of those email addresses are going to work, I think one may as well just consider the local part to be any string of Unicode code points.)

Think about this: it is up to your SMTP library to correctly escape the email address you give it for SMTP, and here's the fun part: if you give it a pre-escaped email address, then clearly it is going to have to double escape it, right? So, semantically, these extended discussions of quoted strings and character limitations are always just so ridiculous :/... you absolutely should not be dealing in SMTP-escaped addresses or asking your user to understand SMTP (and the same goes for MIME).

(BTW, if you want some "real hell", one of these two protocols--I forgot which... I presume SMTP--seriously supports an empty local part. If that doesn't tell you everything you need to know about un-opinionated these RFCs are with respect to "anything goes" then I don't know what will ;P.)