If you copied your "corrupt"-mbox-folder (that's the file without an extension) into your Local Folders - directory but use, for that folder, a unique name.

If the mbox isn't really corrupt then you can move what ever mail you want to where ever you want.

If its corrupt you can try to repair folder under folder properties (right click on folder)

Thanks Gnospen, I renamed "corrupt" -mbox-folder as InboxOld and copied into the Local Folder. The received mails were accessible and have copied them into the current inbox.

I still cannot see the sent mail associated with the "corrupt" mbox file.

There in there own file/folder "sent items", directory depending on what you set up in account settings /copies & folders

You should read this about keeping mails in your inbox. http://kb.mozillazine.org/Keep_it_working_%28Thunderbird%29

The sent folder for the backed up mails is intact. This is in /LocalFolder/Sent . The sent mail from the "corrupt" InboxOld file is not seen.

Thanks for the link. After reading the link I have adopted some of the best practices mentioned there.

I dont know how your setup was. But it's NOT in Inboxold-file as that is a file for incoming mails. There could be a second profile on your C-disc C:\%appdata%\thunderbird\profiles. And your sent folder is in "profilename"\mail\"account" Or "profilename"\mail\local folders

I think the d:\mails\Sent file had got deleted when I set the incoming server to IMAP by mistake. I cannot see the Sent file associated with the inboxOld file. The email backup had the older Sent file, which has been restored. Maybe I will try a file recovery program. Since there was a backup, I have lost only a few sent mails over two days.