>some NHS systems in the UK (one of their target audiences) refused to accept emails that finished with .healthcare
not suprising at all, I've seen e-mail address validation code that (among other things) used a fixed list of TLDs. I could imagine some spam filters doing a similiar thing.
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.
not suprising at all, I've seen e-mail address validation code that (among other things) used a fixed list of TLDs. I could imagine some spam filters doing a similiar thing.