[horde] Dynamically set IMAP server

[Kcam kcam]

I have multiple servers and what I am trying to do is dynamically set the mail server based upon the user's domain. For example the user test at mydomain.com the domain  would be taken and  mail.mydomain.com would be set for the auth mail server. 
I've successfully done this in horde3  but I'm having no luck in horde4 
Here is the code i use from horde3 from /horde/email/lib/Auth/imp.php

list($user,$domainname)=split("@", $userID, 2);$maildomain="mail.".$domainname; $_SESSION['imp']['server']=$maildomain;