[horde] Horde + Courier-imap + exim + mysql : virtual domain

[prince]

I have horde 2.2.4 installed and have courier-imap + exim + myqsl all working.

I have domains like a.xyz.com, b.xyz.com, c.xyz.com etc.

Is there any way, to suffix the domain name with the userid, when the user 
access the mail thru these domains.

if I access the mail by htttp://a.xyz.com/mail (or may be mail.a.xyz.com)
I want the username "prince" to be passed as "prince at a.xyz.com"

if I access the mail by htttp://c.xyz.com/mail
I want the username "prince" to be passed as "prince at c.xyz.com"

I have the userid/password stored in the mysql db as.

userid                        password
--------------------------    -----------
prince at a.xyz.com              W;sdajfDkkkdsf
prince at c.xyz.com              X'wjfwwdfcldsh

Currently, I have the enter the username as "prince at a.xyz.com", but I want 
the domain name to be suffixed by default. Note, the use just needs to enter 
the username (ex, "prince", and when it is passed to the db for verification, 
it will be "username"@virtualdomain.)

Thanks,
prince.