Had this issue recently with one of our smaller clients running SBS 2008 and Outlook would not connect over http from outlook.
First I used www.testexchangeconnectivity.com and selected RPC over http and it was showing 503 errors
Jumped onto the SBS box and went through IIS found SBS Web Applications and noticed RPC and RPCCert was missing
To re-install you need to do the following (if you don’t it will re-install but under Default Web Site as I found
1. Open Server Manager and go to Features > Remove Features > Uncheck Rpc over HTTP Proxy
2. Make sure we don’t have Rpc and RpcwithCert Virtual Directories anywhere in IIS. (Check under Default Website and SBS Web Applications)
3. Open registry editor and browse through HKLMSOFTWAREMICROSOFTRPCRpcProxy and create a string ‘Website’ — Assign ‘SBS Web Application’ as the value to this key.
You may need to create a key called RpcProxy as it will remove this when you uninstall RPC over HTTP.
4. Install Rpc over HTTP Proxy feature through server manager again. You should have Rpc and RpcWithCert VDirs under SBS Web Applications website.
5. Restart Exchange Active Directory Topology Service and do a IIS Reset
Make sure you can browse rpcproxy.dll with a blank page. (Go to IIS, SBS Web Applications and Browse)
6. Run these commands to make sure Outlook Anywhere is working:
Enable-OutlookAnywhere -Server:<servername> -ExternalHostname:<ExternalFQDN> -ExternalAuthenticationMethod:NTLM -SSLOffloading:$true
or do this via the EMC.