When I was validating myself for Amazon Prime Student, I literally had Amazon refuse to accept my student email in the form [email protected] because there were two '.'s in the mailbox portion. I had to send an email to support and it was eventually dutifully fixed.

And that's not an uncommon format for, you know, school emails. And that's an Amazon engineer who should have known.

I imagine there's developers who think "domain.tld" is the only thing valid to put in the domain portion, and that's going to fail with "domain.co.uk", or uncommon TLDs, or other perfectly valid constructs. And sure "it's only x% of the users" but it's a pain in the ass if you're that user. You need to be reasonably permissive.

(but on the other hand "myname@..." is not valid either, and that will fail and cost you money as well... hence leading us back to 'just follow the spec')