Everything preceding the '@' is governed by the rules imposed by the local systems handling email for that domain.  In the past, when the internet was predominantly infested with just geeks, most rules were to be case sensitive.  As more and more 'normal folk' came on it became increasingly obvious that case-sensitivity on that part of the address caused more problems than it solved.  Who wants their email bouncing because someone send it to [email protected] instead of [email protected] ?