I haven't tested this but i think it will work...
look for
// lowercase username if it's an e-mail address (#1484473)
if (strpos($username, '@'))
$username = strtolower($username);
in either program/include/rcmail.php (in the login function) or program/include/main.inc (in the rcmail_login function) depending on what version of RC you have and comment it out.