auto-selection of servers

[Mike Knell]

All,

I'm in the middle of setting up IMP as a replacement for our existing 
webmail system, but we have one sticking point that's causing me something
of a headache:

The system is set up and working fine. One thing we like to have, though,
is a lookup file that lists a default server for each user, along with the
username the system needs to use to log in -- some users are served from
UNIX, which isn't a problem, but others are served from Novell boxes using
Mercury (which I know nothing about) and hence need long-form usernames
(such as .user.foo.bar.baz).

Here's the question -- where should I plug in the bit of code (which we
originally wrote for our old Silkymail installation) to do this correctly?
I've had a look around login.php and redirect.php and nothing exactly
jumps out of the page.. presumably it's just a case of setting $server_key
at the right point, but quite where (it needs the username to be already
available, of course) is somewhat eluding me..

Thanks a million in advance for any assistance,
Mike