Funnily enough the reason unusual email addresses were blocked in a lot of early CGI scripts was precisely to stop the wacky relaying and routing that the autho

Funnily enough the reason unusual email addresses were blocked in a lot of early CGI scripts was precisely to stop the wacky relaying and routing that the author points out could possibly happen given a sufficiently ancient mail setup. See e.g (PDF warning): http://www.city-fan.org/ftp/contrib/websrv/formmail-advisory.pdf http://www.city-fan.org/ftp/contrib/websrv/formmail-advisory... "Creative Recipient Addressing" People forget what a big problem spam used to be. And the utter insanity that ensued when you combined Perl CGI scripts with poorly configured Sendmail on dodgy hosts. I think "^[A-Z0-9+_.-]+@[A-Z0-9.-]+$" or similar got enshrined as "best practice" sometime back then and has survived to the present day by framework inertia and copy/paste transmission from sites with 10+ year old answers to "how to validate an email address". Now we've gotten to the point where it's probably OK to try to send to an "emoji cat face" or "<dog=cat>"a%b@c!d.e@foo.com but nobody really expects those to be reliably deliverable or usable for signup anymore. It's just a UX thing now. My observation is that if you use the dumb obvious regex but just give people the "+" in the local part, nobody will complain. Certainly not by email, anyway...