Have a read of RFC 2822[1] section 3.4

In brief, the address specification may look like the simple "local" @ "domain", but those subparts can be non-regular (i.e., making them hard/impossible for a regular expression engine to parse) or contain a lot of exceptions (e.g., the domain could be google.com, or it could be 12.34.56.78, or localhost, or a number of other things).

[1] https://www.ietf.org/rfc/rfc2822.txt

It's not a regular grammar. Regular expressions were designed to handle regular grammar, and there are cases where something that looks valid is actually invalid: ex@256.255.255.255.

(See the above posters link: https://nikic.github.io/2012/06/15/The-true-power-of-regular...)