lmhost for im to exchange server

[Mark IM Weakling]

My Company recently disabled all IM outside of our
network. We are firwalled so I use a vpn to connect
remotely to our network. How do I set up a lmhost file to
enable myself to connect to our IM server. I think that
is why i get this message when trying to sign
on. "signing in to microsoft exchange instant messaging
failed because of a problem connecting to the service"
How do I get past this. Thanks for any info