The era of known top-level domains or valid TLD patterns is mostly over
Once upon a time you could create a list of valid top level domains and
thus rapidly validate if an email address presented to you was even
possibly correct. If you were slightly more bootleg you could do it
with patterns, since all TLDs were either two letter country codes or
three letter 'traditional' TLDs (and you could actually list those).
This era started to crumble a while after I did my bootleg hacks to
a mailer here, with the introduction of things like
.info, but for
a while I patched up the cracks by accepting four-letter TLDs.
That era is over now. It's probably been over for a while but I didn't
really notice because new TLDs haven't generally been introduced or
at least used in email to this one ancient system that basically only
gets spam now. But I just got an email attempt with a
MAIL FROM of a
.travel domain and the domain actually exists so, well, so much for
that. So I've taken that particular bit of bootleg cleverness out of
this old mailer's configuration.
(I don't regret rejecting this particular email message because, as mentioned, it's highly likely that it was spam. But I sort of want this mailer's rejections to not be laughably wrong.)
PS: I could probably arrange to get a list of all currently valid TLDs from somewhere if I really wanted to. But that's more work than this old mailer justifies.
(Why I care about detecting valid top-level domains instead of just checking whether the address is fully valid is a long, sad story involving some design decisions by this mailer that were valid when it was written a long time ago but aren't really any more. The short version is 'uncontrollable address canonicalization'.)