After a mail server failure, where mail data is lost, it may be possible to recover a user's mail from the Thunderbird local IMAP cache.

1. DO NOT reconnect Thunderbird to the new mail server.
   • If you do, you may lose everything!
2. Set Thunderbird to Offline Mode
   • File → Offline → Work Offline
3. Copy (drag and drop) the still-visible mail and mail folders from the IMAP account into Local Folders

Once you are confident that all the desired mail has been copied/moved to Local Folders:

1. Set Thunderbird to Online Mode
   • File → Offline → Work Offline (again)
2. Connect the new mail server account
3. Copy or move the mail from Local Folders to the new IMAP folders