If your emails are displaying in the email client called 'Outlook' which is installed on your computer then you could try: In Thunderbird Tools > Import > Import from Outlook - click on 'Continue' and follow instructions. OR see if Outlook can export the messages in .eml format to a folder on desktop and then use this addon 'ImportExportTools NG' to import those .eml files into 'Local Folders' account. You cannot directly import back into an imap account, but once you have them in Local Folders either leave them in that location or select/highlight emails and use 'Right click' > 'Copy to' > select imap folder. This would upload to server, but you will need to do it in small batches. Addon called 'ImportExportTools NG' https://addons.thunderbird.net/en-us/thunderbird/addon/importexporttools-ng/ How to install an addon: https://support.mozilla.org/en-US/kb/installing-addon-thunderbird To import: Create suitable folder in 'Local Folders' account Right click on that folder and select 'ImportExportTools NG' > 'Import eml messages > 'All eml messages from a directory' and then you get directed to select the folder which contains those eml messages.