re :I changed my account from pop to imap, and then back to pop again

When you say 'changed', I hope you mean, you created a new imap mail account, then deleted it and created a new pop mail account. You cannot simply change the server settings as they are two entirely different setups.

When you remove a pop mail account, it is not actually deleted from the profile folders unless you manually chose to do it. This means when you create a new pop mail account you can redirect it to look at the original pop mail account. So, if the account is still in original location try this:

• Right click on pop mail account in Folder Pane and select 'Settings'
• Select 'Server Settings' for pop mail account.
• Bottom right you will see 'Local directory'
• click on 'browse' button and locate the original pop mail account which will be in the same profile folder as you are currently using.
• click on 'OK'

• Restart Thunderbird.

Additional info: http://kb.mozillazine.org/Thunderbird_:_FAQs_:_Recover_Deleted_Account

If you completely removed the original Thunderbird folder from the appdata file profile name folder, thus deleting the original profile name and accounts, a new profile name folder would have been created. You could import the 'Sent' no extension file from the 'recovery' location you show in the image. Use an addon called 'ImportExportTools'.

• https://addons.thunderbird.net/en-US/thunderbird/addon/importexporttools/

• Install the addon and restart Thunderbird.
• Right click on pop mail account and select 'ImportExportTools' > 'Import Mbox file'
• select 'Import directly one or more mbox files'
• click on 'OK'
• locate the 'Sent' file as shown in your image
• click on 'Open'.
• File should get imported.

Thank you very much for the directions, but the first step did not bring the Sent items back, although it did bring all the Inbox messages in. The second suggestion did not work because the add-on is not compatible with my version of TB. Any other tricks?