Hi,sjs4

This scenario is incredibly frustrating, but based on your description running Thunderbird on Linux Mint, it is likely a folder indexing error or an IMAP synchronization glitch rather than a permanent deletion of the email. Because you confirmed that the mail was visible in the new folder before shutting down, the data was successfully written to your disk. The issue is that Thunderbird can no longer index or display it. Here are the steps to recover your email and fix the folder: The "Repair Folder" Fix (Most Likely Solution) This forces Thunderbird to re-index the folder content from the actual file on your hard drive. Open Thunderbird. Right-click the new folder where the email disappeared. Select Properties. Click the Repair Folder button in the General Information tab. Click OK. Click on a different folder, then click back on the new folder to refresh. Check for "Filtered" View Sometimes Thunderbird hides emails if a quick filter is active. Click the folder. Look at the top of the email list for a "Quick Filter" bar. If any buttons (like "Unread" or "Starred") are highlighted, click them again to turn them off. Check for Folder Subscription (IMAP) If you are using an IMAP account (Gmail, Outlook, etc.), the folder might not be "subscribed" to the server. Right-click the account name in the left panel. Select Subscribe. Ensure the new folder has a checkmark next to it. Delete the Index File (.msf) If repair fails, you can force a rebuild by deleting the index file. Close Thunderbird. Navigate to your Thunderbird profile folder (usually ~/.thunderbird/ in Linux Mint). Navigate to Mail/[AccountName] or ImapMail/[AccountName]. Find the file that matches the name of your new folder, but ends in .msf (e.g., ImportantStuff.msf). Delete this .msf file. Restart Thunderbird. It will automatically recreate the index. Check Webmail (The Source of Truth) Log into your email account via a web browser (e.g., mail.google.com). If the email is there, the issue is strictly a Thunderbird synchronization problem. If the email is not there, the move process failed during shutdown, and the email might be in the Trash or All Mail folder of the server. Why this happened Improper Shutdown: If Thunderbird was still syncing with the server when you shut down, the index (.msf file) could have corrupted. Move Crash: Moving thousands of emails (or one, if the index is large) can cause issues if the software crashes. Linux/Thunderbird Sync: Sometimes, Thunderbird requires you to click on a folder to download its full content. To prevent this: Regularly Compact Folders (Right-click folder - Compact). This prevents large index files from becoming unstable.

@sjs4,

What did you find after Miran's information?

Is the lost message being copied from an imap account? And is the message being copied to a local folder?