Unless they're in the trash folder, they're gone.

If you were using an IMAP mail account, it can only display whatever is on the server as typically viewed if you used a browser to logon to your webmail account. It you have a backup then you could use it. Note: If using an imap mail account, you have to import the backuped mbox files into 'Local Folders' mail account. Ideally, just leave them there as you will still see emails. But otherwise, highlight small batches of imported mail seen in Local Folders and use the right click on highlighted mail > Copy to > choose imap folder. This will gradually upload emails back onto the server. Please note this may take a while because most server have a daily limit on how much you can upload back onto the server via imap. Whatever you do - do not compact anything at this point. You could try to recover emails - easier if a pop account, but still possible if imap. Worth a try to see if there are still traces of marked as deleted and hidden emails. In Thunderbird * 'Help' > 'Troubleshooting Information' * click on 'open folder' a new window opens showing profile folder name * Close Thunderbird now - this is important * If this was a pop account - click on 'Mail' folder * click on pop mail account name or 'Local Folders' - whatever has missing emails. Additional info that might be relevant' for imap accounts. * If this was an imap account - click on 'ImapMail' folder * click on imap mail account name that has missing emails. In this example I'm talking about the Inbox, but same applies to any folder name. You should see the 'Inbox' mbox file - it has no extension. I'm not sure what folder the deleted/lost message was originally in, but there will be an mbox file for that folder name. * Open the no extension mbox file using Notepad++ Emails are written to the file one after the other, so oldest will be at the top and newest at the bottom. Each email will start with these lines: this is an example * From - Sun Dec 28 18:14:40 2014 * '''X-Mozilla-Status''': 0001 * X-Mozilla-Status2: 00800000 * Use 'Edit' > 'Find' * search for: '''X-Mozilla-Status:''' Look for this line: X-Mozilla-Status: (not X-Mozilla-Status2:) Typically emails that have been moved/deleted will have these numbers : 0019 or 0009 So as you work down through file - if you see emails with those number - edit them. * Make sure the '''X-Mozilla-Status:''' has the number '''0001''' * Save the file. * As you have edited the 'Inbox' mbox file - delete the 'Inbox.msf' index file. A new one will be auto created. The above works ok for pop accounts and the 'Local Folders' mail account. IF you are talking about an Imap mail account then it's likely emails do not exist on server so you need to perform an additional action. Do this if the file was in an imap account. Perform the same 'X-Mozilla-Status' process as mentioned above. * Copy the saved mbox file. * Paste the saved file into the 'Mail' >'Local Folders' mail account folder. * Then rename file eg: rename as 'OldInbox' if necessary. Restart Thunderbird. Any pop emails should reappear in pop account. Any Local Folders emais should reappear in Local Folders account. Any rescued imap emails should reappear in the Local Fodlers account with modified name for folder.