Well, Sharkfin, I have to agree with you.

The user doesn't have to "fix" what is incorrect behaviour from the application. A message not sent should nor be marked a sent, dot.

If I click to start a reply to a message but then cancel that message without sending it (or even saving it), the original arrival is still marked as having been replied to.

This should not be done if the reply is cancelled. In fact it should not really be marked as such until the reply has been transmitted to the outgoing mail server.

Right-click at regarding message and select Message properties... (F12) and deselect 'Has been replied to'.

It's OK - I know how to fix it. This is a suggestions/wishlist posting, not a support request. I'm just saying that the program would be better if the fix were not required.