[imp] IMP + Courier-IMAP: Problem with virtual domains

[Christoph Mühlmann]

Hello,

I'm using horde3 3.1.6 and IMP 4.1.4 together with an working
installation of courier-imap plus the other courier components from
Debian Stable/Etch. 

Within courier I use virtual hosts, so users from virtual hosts
("locally-hosted domains") have acces to their mail via IMAP with
user-id's like "username at virtualhost.tld" or "person at host2.tld" while
server-users ("local domains") have access via their user-name on the
system. The default domain, which is set in courier is
"defaultdomain.tld". This works fine with sqwebmail or squirrelmail.

The Horde/IMP-Installation should only be reachable via one single
Webserver (mail.defaultdomain.tld), i don't want to set up additional
hosts like mail.virtualhost.tld or mail.host2.tld, only one
IMP-installation on one webhost for all users.

in /etc/horde/imp4/servers.php:
$servers['mail.defaulthost.tld'] = array(
        'name' => 'Default Host',
        'server' => 'localhost',
        'hordeauth' => 'true',
        'protocol' => 'imap/notls', //'imap/ssl',
        'port' => 143, //993,
        'maildomain' => '',
        'smtphost' => 'localhost',
        'smtpport' => 25,
        'realm' => '',
        'preferred' => '',
        'quota' => 'courier'
);

Those courier-imap-user-id's are also the user_uid's in horde, which
means person at host2.tld shoul be able to log in as person at host2.tld at
the horde/imp-installation at mail.server.tld.

Now my Problem:

person at host2.tld logs into horde and gets his usual welcome-page. But
when she/he wants to read her/his emails with IMP the login to the
imap-server fails, IMP tries to login with person at defaultdomain.tld,
wich doesn't exist, instead of person at host2.tld. 
It seems Horde or IMP strips the domain-part from the user-uid and the
default-domain is added by courier or IMP. (It seems that courier adds
the default-domain, because after the authdeamon tries
person at defaultdomain.tld via auth-mysql, wich fails, it tries only
"person" with auth-pam)

I had a look on /etc/horde/horde3/hooks.php but found not what I
wanted. 

Perhaps somebody knows an easy solution for my problem?

Thanks,

Christoph Mühlmann