Comment by jandrese
Email is one of those cursed standards where the committee wasn't building a protocol from scratch, but rather trying to build a universal standard by gluing together all of the independently developed existing systems in some way that might allow them to interoperate. Verifying that a string a user has typed is a valid email address is close to impossible short of just throwing up your hands and allowing anything with a @ somewhere in it.
Email is one of the very few success cases of the xkcd Standards meme: https://xkcd.com/927/ - and it's due to practicality and ingenuity on the part of people who made very creative parsers and placed real-world understanding behind every word of the early RFCs.
Without a unified email standard, the world would look incredibly different today, especially as it bootstrapped open communication between different countries and institutions in developing every protocol since.