OPS!
I´m sorry I don´t really see your message on General Phorum! OPS!
BTW I have done some forum search:
Re: Webmail defaults to "admin" 
Author: Aavo Petrauts (aavo_AT_karu15.ee)
Date:   04-03-02 06:43
I am edited the row in the file:
/home/httpd/html/horde/imp/templates/login/login.inc
 | and just deleted
value="= Horde::getFormData('imapuser', getenv('REMOTE_USER')) ?>" 
I am not see any reason, why it must be there.  
This is from: 
http://forums.contribs.org/index.php?topic=13369.msg50592#msg50592