Hello Vincent,
You are on the right avenue. The current forum is for developers. I'd suggest asking non-programming questions on the Outlook IT Pro Discussions or Microsoft Community forums instead.
> When I move a new e-mail to the sub folder it does not recognise it as the same conversation as e-mails already present, even though the subject is exactly the same.
First of all, make sure that you set up the current view to group Outlook items into conversations.
Conversations are based on the time and date marker and the subject line. You may check out the ConversationID property of Outlook items to make sure that items are not changed. You can use any low-level property viewer for observing item properties at runtime - see MFCMAPI or OutlookSpy.