Can't get to Sent Messages via backward arrow icon

Question

When selecting the backward arrow icon in the mail column view I always get an error message reading 'Mail was unable to find your response to the message “message title”. You may have deleted the message.'
This occurs ALL the time and with ALL messages I've sent, even right after sending an e-mail. What is the problem here? I certainly did not delete the copy of the outgoing message. It still always can be found in the sent message mailbox.
Can anyone help?

Answer 1

Ernie
Yes, I have (and on rare occasion use) webmail access to my IMAP account.
I mentioned the bit about the system generated Sent, Trash and Draft folders to indicate that I'm aware that the Sent (etc.) mailbox that shows up under my account in the sidebar is a different mailbox, generated automatically by the mail server. It is unused (and remains empty) except for when I compose and send messages through the webmail interface (which is so rare as to be virtually never).
I can see why it might be thought that Mail is getting confused between the Sent mailboxes (i.e. they seem to share the same name and to serve a similar function), but I can't really see why Mail actually would get confused: If an application can at one time recognise the difference between two folders, why not at all times? And wouldn't the threading function be consistent in the way it searches for replies (or annotates them for future reference) such that it would either always work or never work (since one of the sent mailboxes never contains messages)?
(That said, I'm more than ready to accept that my inability to see why Mail would get confused is owing to my relative ignorance about how these things work.)
In any case, Mail is saving sent mail to the Sent mailbox stored on the HD and never to the Sent folder on the server.
"Command-I" on the Sent mailbox in the sidebar shows the same settings for mailbox behaviour as is shown in Mail preferences.
What I find bewildering about this quirk is not so much that the threading doesn't work, but rather that it works inconsistently. Then again, that's probably because I am, as with most lay-users, fairly incapable of appreciating the enourmous complexity of cross-reference (etc.) underpinning what appear to be simple actions whose effects are expected to be uniform.
Forgive my speculative digression, Ernie, and thanks for your continued assistance.
MBP 2.0GHz 2GB 120GB Mac OS X (10.4.7)
MBP 2.0GHz 2GB 120GB   Mac OS X (10.4.7)