This line comes from horde/imp/templates/login/login.inc :
<input type="text" tabindex="1" name="imapuser" value="<?php echo
htmlspecialchars(Horde::getFormData('imapuser', getenv('REMOTE_USER'))) ?>"
/>
Why there is a value given to this imapuser field?
All users of our Horde system get the user field filled with my user name!
Patrick Atlas