Exchange, like almost all of Microsoft's products, uses closed
specifications. Therefore, all open source products have to
reverse-engineer the communication protocol. Try googling, but don't
expect too much.

Presumably any Java<->COM bridge would be able to talk to Exchange usingf MS's
COM or Axtive/X APIs.  I don't know whether you'd need two way communication
(there might be COM events you'd want to listen for -- I dunno).  I'm not
convinced by the requirement for "open source" software (if you are using
Exchange at all then there is presumably money around to pay for licenses....),
but there are open source COM interfaces for Java.

Some names I turned up a few days ago when I was looking at COM/Java stuff:

   com4j
       http://com4j.dev.java.net/
   ComfyJ
       http://www.teamdev.com/comfyj/index.jsf
   EZ-JCom
       http://www.ezjcom.com/
   Jacob:
       http://sourceforge.net/projects/jacob-project/
       http://danadler.com/jacob/
   jacoZoom
       http://www.jacozoom.com/en_jacoZoom.shtml
   Java2COM
       http://www.nevaobject.com/_docs/_java2com/java2com.htm
   Jawin
       http://jawinproject.sourceforge.net/
   J-Integra
       http://j-integra.intrinsyc.com/products/com/
   J-Interop
       http://j-interop.sourceforge.net/
   OKEJA
       http://www.javain.com/javain/oleja.jsp

(I haven't used /any/ of them.)

   -- chris