Ignoring the dialog, what exactly is it you think you have lost and want to recover?

The dialog is simply asking to recover space in the storage file for email you already removed from view in it, perhaps by moving them to another folder or even deleting them so they are marked as deleted in the inbox and then not displayed. It is a housekeeping function it is being asked to allow running, but it can impact performance..

What I'd like to recover is a number of mails which I deleted by mistake.

What I'd like to recover is a number of mails which I deleted by mistake.

and they are not in the trash / Deleted mail folder?

All trash/deleted folders are empty. Just now I emptied them again and as a result the dialog window still shows that there is approx 380Mb to delete. This gives me hope that in that volume I can find the mails which I by mistake moved to...

re I'd like to recover is a number of mails which I deleted by mistake - All trash/deleted folders are empty. Just now I emptied them again. If you want to recover emails accidentally deleted then it does not help if you keep emptying the Trash- as you say"I emptied it again".

Do Not compact anything; this is absolutely vital at this point in time.

In Thunderbird

• 'Help' > 'Troubleshooting Information'
• click on 'Open folder' button

a new window opens showing the contents of your profile name folder.

• Close Thunderbird now - this is important because you cannot edit files currently in use.

• If pop click on 'Mail' folder - if imap click on 'ImapMail'
• click on mail account name folder to see contents.

In Thunderbird, emails are stored in mbox text files. mbox text files do not have any extension. if you only see .msf files then you do not have fully downloaded copies.

You should see files that have same name as Folder name. eg: Inbox, Sent etc If you deleted emails from Inbox then

• Open the 'Inbox' no extension mbox file using a text editing program like Notepad

If it does not open because the file is too big then you will need to locate and download a text editor program that can handle very large text files. Perhaps this measn you should not keep too many emails in one single file. Create subfolders to organise emails and it will help to keep a smaller file. Compacting regulary will also keep files small, but you cannot recover from a file thats been compacted because the deleted files will no longer exist. So do not empty Trash if it contains something you want.

Emails are written to the file in the order they are downloaded or created, so the oldest will be at the top.

Each email will start with these lines: this is an example

• From - Sun Dec 28 18:14:40 2014
• X-Mozilla-Status: 0009
• X-Mozilla-Status2: 00800000

I've included a couple of example images to help show you what I'm talking about.

• Use 'Edit' > 'Find'

Starting at the top use 'Edit' > 'Find' to help to look for this line:

• X-Mozilla-Status: ( - NOT this line: X-Mozilla-Status2:)

Make sure each X-Mozilla-Status: line has the number 0001

• Edit the number as required changing to say 0001 working down through the document. Take your time.
• Save the file.
• Delete the 'Inbox.msf' indexing file (or whatever corresponding .msf). A new one will be auto created.
• Start Thunderbird.

Work through the Inbox and delete emails you do not want. Organise emails into suitable folders for storage. Then compact the Inbox folder to tidy it up.