The local part of an email address is not required to be case-insensitive, except for the Postmaster address. RFC 822: "The domain-dependent string is uninterpreted, except by the final sub-domain; the rest of the mail service merely transmits it as a literal string."

However, there are too many services smashing the case of the local part to either lower or upper case that case-sensitive names have become nigh impossible to use.