Hi Simonsham,
due to many various types of applications communicating on port 80, we decided that only browsers should be scanned. There is so many different uses of HTTP protocol, that incorporating HTTP proxy (such as WebShield) in all connections might bring troubles from time to time.
However, every user can configure webshield to scan what ever he or she wants. In your case, just edit avast4.ini, section [WebScanner] and add the line: OptinProcess=outlook.exe (or add ",outlook.exe" - without the quotes - to what ever already is on the Optin line).
lukas.