If you changed ISP and emails were on the server of that old ISP and were using an IMAP mail account which synchronised with an empty account then emails would get deleted because IMAP folders can only show what is held on server. If you have a backup and that backup contains mbox files, then you could get copies of the mbox files and put them into the 'Local Folders' mail account. There are two methods available: By addon or manually There is an addon called 'ImportExportTools NG' which can be used for importing mbox files into 'Local Folders' mail account. https://addons.thunderbird.net/en-US/thunderbird/addon/importexporttools-ng/ How to install addon: https://support.mozilla.org/en-US/kb/installing-addon-thunderbird '''To do it manually:''' InThunderbird Help > Troubleshooting Information Under 'Application Basics' locate 'Profile Folder' and click on 'Open Folder' This opens the 'profile name' folder and shows the contents. Exit Thunderbird now - this is important. Click on the 'Mail' folder to open Click on 'Local Folders' folder to open This is where you need to get copies put. Access backup 'profile name' folder In 'ImapMail' folder is imap account name folder which may contain mbox files, but only if you synchronised folders to download full copies. mbox files do not have any extension. files with .msf extension do not contain emails Folders with .sbd extension are created to contain subfolders - which may contain mbox files. you would need to get copies of all mbox files and .sbd folder put into the 'Local Folders' mail account which is in the 'Mail' folder of your 'profile name' folder. See image below as guide.