My favorite is e-mails with three dots in them. Which is actually not a valid address - the RFC specifies that you must have a valid textual character between dots[1]. However, because of poor decisions by Japanese telcoms, a substantial chunk of their users have 'e-mails' associated with their mobile phones with three dots, breaking goddamn every sensible validation script.
Sadly, if you're sending e-mail sanely, your mail provider likely validates recipients, and will be annoyed at you if you send them recipients they think are bogus.
The relevant rfc (on mobile; don't remember which) specifically states that intermediate servers must not validate mailboxes (local parts). And honestly the domain should be "validated" by the server doing an mx lookup; let dns handle it.
So wait, what now? So you can have an email address like first.last@isp..co.jp? Can you give me a generic example?
Yeah, I'm with the other guy, regardless of whether or not it's a good idea to do validation (it's not), that's not an address that should pass validation because it's not a valid domain or hostname.
I could see it being less of a big deal in the mailbox portion given that it's now kinda kosher to ignore dots there.
[1] https://tools.ietf.org/html/rfc2822#section-3.2.4