Instant Messaging client-server solution?

[Jaap Winius]

Hi folks,

Can anyone recommend and an Instant Messaging solution, client and  
server, that plays nice with Kerberos 5?

The group of people I would be setting it up for all recently switched  
from using Windows XP workstations to Debian squeeze with Xfce.  
They're still getting used to the environment, so I don't want to  
offend their sensibilities too much with an IM client that is too  
minimal. They currently would prefer to use Pidgin, but are still  
flexible.

Their network consists of three geographically separate locations,  
each with its own Debian squeeze server that includes an iptables  
firewall and NAT, as well as IPv6 (and another firewall for that). The  
three servers communicate with each other via the Internet, but always  
through the firewalls (and NATs for IPv4). Zephyr may be a solution,  
but I'm not sure it would work with the NATs.

Thanks,

Jaap