Why doesn't reply arrow find my reply?

Question

When I click the curly arrow which should take me to the reply that I typed to an email, it tells me what it can't find the reply and that I may have deleted it.
However, when I check in the Sent mail folder, there is my reply. Why can't Mail find it?
I have rebuilt both the InBox and Sent Mail folder, but it still doesn't find it.

Answer 1

Hi Ernie
Thanks for your replies. I meant to say yesterday too Thanks for the Welcome - I've been here before too, but when I came yesterday I was required to re-register - some change in the forums I guess?
Yes, I did run under Panther, and for some of my mailboxes under Jaguar too.
I've looked in all the various folders in the ~/Library/Mail/ folder and frankly have been puzzled by what I find.
Yes there are other files in some of them, that obviously aren't being used now as the modification dates are not recent, so I'd assumed that the various updates to Mail along the way have made some files redundant. Can these files that aren't either info.plist or the Messages folder be safely deleted?
I also can't see a consistent way that the various folders are setup and named. There are folders outside the Mailboxes folder that seem to be being keptup and current, but they don't seem to match all the accounts, nor the folders in the Mailboxes folder.
For example I have 3 AOL accounts handled by IMAP. In the Mail folder two of them are identified as "[email protected]" and "[email protected]" but there isn't a folder named for my primary AOL name RogerKIWI.
In Mail.app all three IMAP boxes have the same grey world icon, and are similarly named [email protected], [email protected], [email protected] so why aren't the folders in Library/Mail similarly named? Yet apart from the oddities of not always being able to open Replies it does all seem to work.
I've put a screen shot showing the two folders contents here
http://LisaAndRoger.com/Mailboxes.png
I'm wondering if "starting over" might be the way to go - if I make a duplicate of my Mail folder so everything is saved, what would I need to keep in order to not have to set up my Accounts, Rules and mailbox hierarchy in place, then import the messages into them in the hope that from now on this "show me the reply" button will always work.
Is the information about the Replies kept with each message, or is it kept in a central data file somewhere? What does the "Envelope Index" file keep track of?
Thanks
Roger
PowerMac G4 Dual 1.25 GHz   Mac OS X (10.4.4)