Microsoft introduced RPC over HTTP as new feature for Exchange, but configuration of this feature not an easy task. I have experience few problems when configuring this, one of the main problem I had face is publishing RPC over HTTP through ISA 2004.
Here I configured RPC over HTTPS using Microsoft certificate service, it works well in LAN but couldn't able to access from out side after publishing through ISA 2004.
Solution for this is add record to ISA host file using internal IP of front-end server and external domain name. Then publish Exchange front-end server using external domain name it works well because outlook can maintain FQDN.