First of all i'd like to say RC is the ebst webmail i've worked with til now. So, switching to it i needed to let my users the freedom of logging in only with the first part of their e-mail addy. It's not possible by default. Anyway after searching everywhere possible (less forums since they are not working at this moment) i got to look into the code myself. Therefore i decided to make use of a main.inc.php variable $rcmail_config['mail_domain']. So i changed index.php like this:

Make this line: else if (isset($_POST['_user']) && isset($_POST['_pass']) && rcmail_login($_POST['_user'], $_POST['_pass'], $host))

to look like: else if (isset($_POST['_user']) && isset($_POST['_pass']) && rcmail_login($_POST['_user'].'@'.$CONFIG['mail_domain'], $_POST['_pass'], $host))