[imp] Override web domain name

[lists at gordonross.me.uk]

My IMP/Horde server is behind a firewall. As such, the Internet domain 
name of the IMP server is different to the hostname of the machine that 
IMP/Horde is running on.

Whenever I login to IMP/Horde, I get sent to the hostname of the machine 
that is running IMP/Horde. Because this does not exist on the Internet, 
the login fails.

Is it possible to tell IMP/Horde to use a different domainname rather than 
the hostname when generating URLs, etc ?

Thanks,

GTG