not a zimbra issue maybe :0)
for now external the zimbra people have disabled external communication via IM. But that would not be what you want: you don't want to use the zimbra-webclient but your corporate directory within the server (the GAL, thus zimbra-ldap).
Zimbra uses openfire/wildfire by Jive software as IM-server. This server can be downloaded and installed stand-alone and supports many (freely-)available clients that can be installed and used next to your outlookclients.. Than just install and configure openfire to use your zimbra ldap and you'd be on the road you want to follow?
Unless you're authenticating to an external source ldap/AD this may work for you.
Last edited by PeterH; 02-18-2008 at 03:27 PM.
Using ZCS Network-edition 5.0.16 on Ubuntu 6.06.2 LTS and 8.04 LTS