Squid, authentication, Outlook Anywhere, Windows 7 and HTTP 1.1 = NIGHTMARE

Windows 7, Server 2008, and I believe even Vista don't support NTLMv1 by default. If it's working on XP, but not Windows 7 I would start by enabling NTLMv1 and seeing if it resolves your issue. Here's the post that helped me when I had a similar issue. Windows7 - “The specified network password is not correct.” when the password is in fact correct

Loading rpcping.exe into Dependency Walker, it's clear that it uses WinHttp.dll instead of WinInet.dll. So, copying the WinHttp.dll from Windows XP SP3 into C:\Program Files\Microsoft Office\Office11 where Outlook.exe is located causes Outlook to send HTTP/1.0 requests with User-Agent and all the other header fields just like it did on my old computer.

Existing version on Windows 7 SP1: 6.1.7601.17514

Copied version from Windows XP SP3: 5.1.2600.6175

It's not the cleanest solution, but it will have to do for now. If anyone has a better idea, I'll all ears...