This really depends on how you have Exchange set up. Are you using RPC over HTTP(S)? If not then are your users connecting by VPN first? If you are not set up for RPC over HTTP(S) then your users HAVE to connect by VPN first or the server will not be available. If they ARE using RPC over HTTP(S) then there are some configuration issues that will need to be resolved. Please provide the details of the connection. Thanks.
<a href=”http://www.eggheadcafe.com/software/aspnet/29817540/articles-on-setting-up-rp.aspx”>Here is an article </a>on rpc over https that may help you.
For troubleshooting <a href=”http://www.msexchange.org/tutorials/Troubleshooting-RPC-over-HTTPS-Part1.html”>try this article. </a>There is also a part 2 if you need it.