RPC over https - Exchange 2003

we have configured the LTM to load balance front-end server http&https. there is no ssl offload

 

 

The users are able to connect to the server (Mail instance only)

 

but when they try to connect to directory it fails.

 

 

that is only happen if they try to connect through the LB

 

when they connect to the server directly it works perfectly

 

 

so any clue why ??!