I am looking for some API to connect to OCS server and send a chat message. I can understand Microsoft uses a modified version of SIP protocol, usually referred to as SIPE.
Pidgin has a plugin to communicate with OCS using the below plugin
http://sipe.sourceforge.net/
But I could not find any Java API to use it directly. Has someone done this before?
There is a simple API lib for this:
http://jsip.sourceforge.net/
It has not been updated for ages, but should be enough for you to get started.
Good luck!