[imp] Suffixing (sub)domain name to userid from horde/imp

[prince]

Dear Group,

I have a single horde/imp configuration file shared by many virual domains. 
Currently my imp/config/servers.php has the maildomain set to a particular 
value (say first.example.com). Now, I want to suffix the userid with the 
domain name, where they are accessing the mail.

Suppose, if the user user1 access mail using http://1st.example.com/mail, I 
want the userid to be passed as user1 at 1st.example.com . If (s)he access the 
mail via, http://2nd.example.com, then the userid should be 
user1 at 2nd.example.com
For user2 it will be user2 at 1st.example.com, user2 at 2nd.example.com etc

I have these users in a common mysql database. Currently the user has to 
enter manually user1 at 1st.example.com. I want the user to just enter his user 
id and based on the domain I want horde to prefix the domain name.

How can I do this? I went thru google, but did not find any complete 
document. Does anyone have a working example using hack or so?

Thanks,
Prince.