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I've had e.g. foo@mysub.mydomain.com rejected by one of the world's top-3 shipping/courier services. This was on a .com domain registered many years ago, so it wasn't a TLD issue. And the subdomain and its MX was also established long ago. But their software finally did accept the mail address after I removed the "mysub." part. Crazy!

I know there was someone in charge of ccTLD, and used just that TLD for personal e-mail (i.e. no dots). That would throw off a lot of validators, but I suppose it also was good to avoid all kinds of robots that were scanning webpages for emails.

And I've seen validation that assumed that TLDs were 2 or 3 characters only.

Oh yes, a simple [a-z]{2,3} in regexp. I'm pretty sure I've seen exactly this on many different occasions

Or their 'modern' version: ([a-z]{2,3})|info)