e.g. It drives me BONKERS how many systems absolutely reject my single-letter email (~"N@domain.com"), which I created specifically to make it easy and safe to type on mobile devices etc. Others will reject the "+" sign, or underscore, or dot/period, or (brilliantly) two periods or underscors, etc etc etc :=/
Writing code that doesn't need to exist that has a possible failure mode of not letting someone sign up at all is just a bad decision. If you're going to write that code, either go through the effort of getting it completely right or soften the failure mode. If you really think the user is somehow mistyping their email with special characters or an unusual TLD, then you could show them a non-blocking warning message.
(Actually RFC5322 already deprecates some syntaxes. For example, "John Hacker"."Ph.D., Esq."@example.com is a deprecated syntax (obs-local-part), because it contains multiple quoted-string components separated by dots.)
> or (brilliantly) two periods or underscors
Two or more consecutive periods is actually disallowed by RFC5322 (unless quoted). foo.bar.baz@example.com is a valid address, foo..bar@example.com is not. ("foo..bar"@example.com is however)
If enough people wrote in about not accepting one letter email addresses, then they would likely update the validation.
But if customer service tells users to use another email address in that scenario and the customer does that, then it might not be worth the effort to fix it.
Better to warm an email doesn’t look right, but let them continue if they want to
When I want to check postage or whatever, and they require an email address, a@b.c typically doesn't work, but no@mail.com does.