Email doesn’t state anything about how you should treat users. User@email.com and user@email.com may be different addresses, but that doesn’t mean you can’t just reserve User@email.com when user@email.com reserves an account. Just don’t send your email to user@email.com when they registered with User@email.com, because the email address is the part you need to leave alone.

If you user modern email services (you probably aren’t, but oh well) this can become a real problem when İ.Akbas@email.tr and I.Akbas@email.tr register. Try to do a caseless compare in your database (select id from users where email ilike ?) and the result will depend on the collation you use.

The Turkish dotted i(distinct from the dotless ı form just one of the reasons you leave user input alone once you’ve filtered out all non-text characters.