punBB with cpanel email (freshmail)

I am using freshmail (freshdevelop.com) that enables me to automatically signup my punBB forum users to get pop3 email from cpanel.
I want to be able to use the forum username as their email name when registering instead of the current default where user can select any email username (and my cpanel email would explode if one user can register 1000x).

This is my attempt:
In the email registration file, I am calling PUN.ROOT from another the mail subdomain i.e. mail.domain.com and the forum is located at forum.domain.com, like this:

    define('PUN_ROOT', '/home/bamboo/public_html/forum/');
    require PUN_ROOT.'include/common.php';

and try to echo the $pun_user['username'] as the email registration name; but now this attempt always give me "Guests" and not the forum user logged in username.

What am I doing wrong?