Isn’t there a TOP and a RETR command involved in downloading messages in Outlook. TOP retrieves the headers information and RETR receives the entire message. I think. Could this be a clue?
Much simpler question - does the problem appear even after reboot? This error code basically mean that the avast engine is not connected to the Outlook plugin. This could happen e.g. if you restarted the avast antivirus service (ashServ.exe) while having Outlook started…
Are your system configurated to Heuristics analysis (Internet Mail Provider)?
avast! can check the HTML part of a message (body). Some viruses make use of bugs in some mail programs (especially unsecured MS Outlook and Outlook Express) that make it possible to start the virus merely by viewing the message in preview pane. avast! checks whether the HTML code of the message does not contain a tag enabling such a trick. If yes, the warning message will be displayed.