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

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@???"

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

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

userid                        password
--------------------------    -----------
prince@???              W;sdajfDkkkdsf
prince@???              X'wjfwwdfcldsh

Currently, I have the enter the username as "prince@???", 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.