Did you enable synchronization, i.e. downloading an offline copy for each message?

yes, now the synchronization is enabled, my problem is that when I checked the "delete all messages except "x" most recent" box, thunderbird immediately shared the information on my imap account and erased multiple thereof thousands of messages. In the library (mac os) I see that a document is very heavy and I guess these are the messages I am looking because a thunderbird download all messages at first and I have any time to empty my trash for example, but I do not know how to find them in my thunderbird interface ...

All mail is in your profile.

http://kb.mozillazine.org/Profile_folder_-_Thunderbird

Look for files without a file extension, e.g. like Inbox.

If you haven't compacted/expunged/purged the folder yet, it may also be possible to recover them without going to the local storage.

1. Right-click on the account in question in the folder list, select "Properties"
2. in the account's Server Settings tab, change "When I delete a message..." to "Just mark as deleted"
3. if they are still around, the messages should now show up in the list with a horizontal bar over them
4. highlight the messages in question with the mouse
5. the "Delete" button should now change to "Undelete", or you can select "Undelete Message" from the right-click menu over the message list

Hope that helps. If not, another option may be the webmail interface on the server, but it appears that you've tried that already without success.

All mail is in your profile. - either way, grabbing a copy of your folders from the ImapMail folder in the profile and storing them at a safe place is definitely a good idea before starting to try anything else, to ensure that you have a backup.

IMAP mail accounts keep all emails on the server.

You 'subscribe' to see emails in specific folders. when you select a folder, those emails are downloaded to a temporary cache to facilitate quicker response in loading. Upon exit, the cache is lost.

If you 'synchronize' folders for offline use or have selected 'Keep messages for this account on this computer' see:

• https://support.mozilla.org/en-US/kb/imap-synchronization#w_configuring-synchronization-and-disk-space-usage

This will download a copy to your computer, but when you delete or move a message from these folders, it will synchronize with the server to update the server. So both folders are one and the same.

If you have not compacted those folders you could see if there are any 'marked as deleted' emails, which are hidden from view.

Read info here:

• http://kb.mozillazine.org/Undelete_a_message

under section 'Edit the X-Mozilla-Status: header in the mbox file' there is info on how to edit the actual mbox file. More info below.

Locate Profile folder:

• http://kb.mozillazine.org/Profile_folder_-_Thunderbird

Once you have opened the Profile Folder, Close Thunderbird before editing any file in Profile folder.

Navigate to:

*   \account name\Inbox

• Right click on Inbox (not Inbox.msf)
• Select Open
• A window opens so you can choose the program.
• select 'Notepad'
• Click on OK

• Click on 'Edit' and select 'Find'
• Type in the following into the search:
• X-Mozilla-Status
• select 'down' and click on 'Find Next'

look for this line: X-Mozilla-Status: Does it have the number 0008 or 0009?

• Change the 0008 or 0009 to 0000
• Keep working through the list using 'find Next'

more info on what X-Mozilla-Status numbers mean here;

• http://www.z-a-recovery.com/thunderbird-deleted-messages.htm

When finished:

• Then click on 'File ' >select 'Save'

Close the Inbox -notepad window by either: clicking on the top right X or File > Exit

Open thunderbird

All the previously deleted emails should now appear as Unread emails.

If you have compacted your Inbox, it will not be possible to retrieve deleted emails.