I came across this error message today on a user using outlook 2007.  After some searching and trying a couple of things I was able to figure out the solution to this error message.  Here is what you need to do.

1. Click Start, click Run, type regedit in the Open box, and then click OK. 
2. Locate and then click the following subkey: HKEY_CURRENT_USER\Software\Microsoft\Office\12.0\Outlook\RPC 
3. On the Edit menu, point to New, and then click DWORD Value. 
4. Type DefConnectOpts, and then press ENTER.
5. Right-click DefConnectOpts, and then click Modify. 
6. In the Value data box, type 0, and then click OK.
7. Exit Registry Editor.

Now open outlook back up and you should be able to access your emails again.  This was a tricky little error and was solved with a quick edit of the registry.  Now if I could only figure out why this happened or what causes this. 🙂