You have to change the ISPConfig code. There is a file interface/web/mail/form/mail_user.tform.php Search for the field "name" (below "password"). Add a new validator entry to the array: 'validators' => array ( 0 => array ( 'type' => 'REGEX', 'regex' => '/^.{5,}$/', 'errmsg'=> 'name_error_needed'), ), (the 5 is the minimum length of the name). Then you have to add a language entry "name_error_needed" to the interface/web/mail/lib/lang/en_mail_user.lng
