I'm happy for my service to quickly and correctly validate 99.999% of emails and I don't really care if your oddball edge case emoji Sanskrit 6-level-deep domain fails. Just do normal stuff.
I once suddenly couldn't log in to a paid bike renting service app because it wouldn't accept my (valid at sign-up) email on login as it had a '+' symbol. There was basically no way to contact the developers and it was impossible to get the support people to understand what my problem was so I literally never used the service again. Don't try to parse email with regex, it's just not worth it.
This occasionally happens to me with a "category.site@depingus.mydomain.com" email address. I also respond by never using that site again.
It has to be a very niche service for you to not have any users with a name that doesn't fit in the pointless constraints of US-ASCII.