The 'windows.old' folder is likely to have old profile info here - assuming all was stored in default location: C:/Users/user name/Appdata/Roaming/Thunderbird/Profiles/profile name/ If emails were in a pop account look in 'Mail' folder and select the pop account name folder If emails were in the 'Local Folders' mail account look in 'Mail' folder and select 'Local Folders' folder If emails were in an imap account look in 'ImapMail' folder and select the imap account name folder Emails are stored in mbox files - they do not have an extension. The files have same name as folder seen in Folder Pane. You need to Copy the 'Sent' mbox file, but if it was a imap sent folder then it could be called 'Sent Items' or 'Sent Mail' or similar. It needs to be put into the current 'Local Folders' mail account, so locate by doing this: In Thunderbird * Menu app icon > Help > Troubleshooting Information * Under 'Application Basics' - about 8th in list - Profile Folder - click on 'Open Folder' A new window opens showing the contents of your 'profile name' folder. * Exit Thunderbird now. * Click on 'Mail' Folder. * Click on 'Local Folders' mail folder * Paste that copied 'Sent' mbox file into the 'Local Folders' folder. Start Thunderbird. You should see that folder containing emails under 'Local Folders' mail account. If you have created an imap mail account, then you could right click on highlighted/selected emails and select 'Copy to' and select the 'Sent'/'SentMail' folder in the imap account. But, putting those emails back on the server will use up server space. I would also advise you do it in batches and be aware that some servers will only allow limited size to upload to server. eg: gmail has a 500MB limit of uploading via imap. https://support.google.com/a/answer/1071518?hl=en However, you could just leave them in Local Folders as you will still have access to them.