From Softros Systems:Softros LAN Messenger is a secure serverless instant LAN messaging program for user-to-user or user-to-group message and file exchange through your corporate network (LAN, WAN, VLAN, VPN). Our LAN Messenger comes both as conversation-like multiple user LAN chat rooms and as classic single-message exchange with the offline messaging capability. Every message passing through your network will be secured by the AES encryption algorithm. Group broadcast messages allow you to notify all users or certain user groups about an event. The office instant messenger program can log your messages to the local computer or to a centralized file server for you to never lose them. Softros LAN Messenger does not require an Internet connection; it works inside your LAN or WAN preventing employees from wasting their time on chatting with their Internet "buddies" via Internet-based IMs, saving your Internet bandwidth and allowing you to minimize firewall breaches that could allow outside attacks and IM worms. Microsoft/Citrix Terminal Services (including the RemoteApp and XenApp application virtualization technologies) and Microsoft Active Directory are supported. Desktop sharing functionality allows you to request remote assistance using Softros LAN Messenger, and the colleague on the other end will securely connect to your desktop to help you configure Windows, install an application, or resolve a technical issue. Mac OS X and Android versions are also available.
We got couple of MS TS servers and linked them using l2tp (2 branches), each of them are running about 120 users on it. We evaluated five intranet messengers available on internet and chose Softros as stable and easy to use tool