It can be problematic when you go to look up the account by email address during login and it isn't found due to inconsistent casing.
It's technically true that the part before the domain can be case sensitive, but as nobody does this the gain in UX from people not having to know the exact casing used during sign-up is worth it to me.
It's technically true that the part before the domain can be case sensitive, but as nobody does this the gain in UX from people not having to know the exact casing used during sign-up is worth it to me.