Configuring where your Sent Mail is stored for IMAP
Thunderbird has an option where you can select where your Sent mail is stored.
1. Click into Tools > Account Settings you will see the following window.
2.Select Copies and Folders on the left hand pane to provide options for storing your sent mail.
3. We recommend you store your Sent mail under IMAP account rather then your local folders by selecting Other. Click on the drop down arrow to select the <account name> of your IMAP mailboxes (e.g Eudora Settings) and Sent Mail.