Alright, guess I'll be using email verification
1) Do nothing and support all valid addresses (that your mail system can handle)
2) Spend development time to make your system not support all valid addresses.
Why do you spend development effort to make your software worse?
4) Realize that the added benefit of supporting weird formats are not worth the time to verify that it actually works.
As someone who works on an email client, I can absolutely tell you that using IP address literals and quoted localparts are more trouble than its worth. Chances are, the libraries you use can't handle them anyways. And if you don't try to support quoted localparts, than normalization is a lot easier. (Although I was once locked out of a system because I signed up with an email address that used capitals and the login form changed to lowercase it without changing the database storage, which meant no form I could spell it would cause it to match).
Hint: I believe domain names cannot begin with digits.
"Be conservative in what you do, be liberal in what you accept from others"
This is a recipe for disaster and code bloat. I met in the past a vulnerability that some irc clients had due to that principle.
A bit of extra effort by the developers saves a lot of people a lot of time.
I have had various email addresses of me rejected because
- of an unusual tld (whatever@somewhere.tech)
- it was an email of a subdomain (whatever@some.where.com)
- it had a two letter username (me@somewhere.com)
... I hope