Allow numbers in email account names

interface/web/mail/form/mail_user.tform.php

@@ -86,7 +86,7 @@ $form["tabs"]['mailuser'] = array (
                     0 => array (  'type'  => 'UNIQUE',
                             'errmsg'=> 'login_error_unique'),
                     1 => array (  'type'  => 'REGEX',
-                            'regex' => '/^[a-z][\w\.\-_\+@]{1,63}$/',
+                            'regex' => '/^[a-z0-9][\w\.\-_\+@]{1,63}$/',
                             'errmsg'=> 'login_error_regex'),
                   ),
       'default' => '',