Most drop-in “is this a valid-form email?” functions do label certain valid email addresses as invalid. In particular, while highly unlikely, it is valid to have an email address at a TLD. Why did this come to mind? Very circuitously. I was watching YouTube videos when I noticed that it was loading stuff from “i.ytimg.com,” which is NOT a valid FQDN. Each part of a FQDN must be at least two characters, with the special exception of the root nameservers. While nearly everyone and everything handles single-character hostnames nowadays, there have been and probably still are some servers that choke on single-character hostnames (I know that I’ve had issues with email addresses with single-character hostnames).
By the way, the combination of these two things means that the shortest email address that will validate in (nearly) all format-validation routines is a single character at a two-character SLD under a two-character coutry-code TLD, such as “a@te.st”.
