perhaps a workaround can eventually fix it.

Make a backup of the profile. Lots of major editing might go pear shaped at some point.

Install the recover deleted messages add-on https://freeshell.de//~kaosmos/index-en.html#recDelMsg

Install the import export tools add-on https://addons.mozilla.org/en-US/thunderbird/addon/importexporttools/

Take Thunderbird offline. (click the two blue computer monitors in the bottom left corner of the window.) No sync so we have some time to work.

Right click the folder in question and undelete. A new sub folder will be created and all available deleted mail will be restored to it.

Right click the new folder and select the import export tools entry. select to export the folder as MBOX. (now the mail is restored and out of Thunderbird.)

Create a folder in "Local Folders to hold the mail. Right click it when selected and again choose the import export tools option, but this time select to import that MBOX you created.

Mail should now be safely located in Local folders, separated from the original account and going back into online mode is safe. (click those monitors again)

The export import is a good way to break the account associations for mail and bypass the somewhat flaky drag and drop of mail from an IMAP account. Sometime it works sometimes it is not satisfactory for large amounts of mail. Never is it appropriate for IMAP where the server is non contactable.

I would suggest anyone using the old server export and import their mail and once done delete the old account from Thunderbird completely as the saga of the missing mail will occur as soon as the missing server occurs. You go an early taste from the user that "changed" the server instead of adding another, but then the old server goes offline everyone's mail will go with it..

Thank you for your insights :)

Unfortunately, the "Recover delete messages" feature does not seem to be working against the saved profile which contains the INBOX file.

Note that, in order to install the plugins, I must restart Thunderbird, and it seems like it has an impact on the mailbox: on first launch, I can see the mails from domain-b.tld (the new one, which the user replaced the old domain-a.tld with), but once restarted, all the mails are gone and I'm left with an empty mailbox. Might be a problem, might not be, I honestly don't know.

re : Note that, in order to install the plugins, I must restart Thunderbird, and it seems like it has an impact on the mailbox: on first launch,

This is because you are restarting Thunderbird in online mode. The files all synchronise with new empty server.

Suggest : Exit thunderbird. Use the backup to put back the original files. then start Thunderbird in offline mode. Use Start > 'Run' If using 32bit type: "C:\Program Files\Mozilla Thunderbird\thunderbird.exe" -offline

If using 64 bit then type: "C:\Program Files (x86)\Mozilla Thunderbird\thunderbird.exe" -offline

click on 'OK'

Then use the installed extensions to do as Matt advised.

Hi,

Thank you again.

What you described is the exact procedure I went through, ensuring Thunderbird is started in offline mode. I am a GNU/Linux user, but for the sake of testing I'm not starting Thunderbird through the CLI. Instead, I make sure to edit profile.ini to remove default=1 (which tends to be automatically added), which ensures I always go through the profile picker where I'm able to start Thunderbird in offline mode.

The mailbox keeps being emptied on restart, after the plugins install. Yet the data is still there (INBOX file still accounts for 1.8Go). Not sure how to install the aforementioned plugin without loosing the content.

re :Yet the data is still there (INBOX file still accounts for 1.8Go).

Before opening thunderbird, access your profile folder imap mail account and create a backup/copy of mbox file and put on desktop; if you have not already done this.

Then start Thunderbird as normal and install the addon extensions as advised.

Because thunderbird is connecting with server for first time, it will end up losing those emails in imap folder because they are not on server. But you have got them backed up somewhere on your computer eg: desktop.

Go into offline mode and use addon extension to import the mbox file into 'Local Folders' mail account. Right click on 'Local folder' mail account and use 'ImportExportTools' > 'Import mbox'.

Once imported into 'Local folders' mail account: Are you able to read the emails or not? If NO cannot see/read emails in the imported folder: then use the "Recover delete messages" on the that imported mbox file.

If Yes, you can read them or you can read them after using the recovery addon: select that imported folder. Right click on a bunch of selected emails and select 'Copy to' and choose the imap folder. Then go into online mode. That should put those emails back on server. Logon to your webmail account to confirm this.

Assuming all is ok, repeat the action of copying emails from Local folders mail account into the imap folder.

Thank you Toal-Hall.

The recovery process, I had understood even before asking for help here actually :)

What prevents me from getting the mails back is simply the fact that, once one has opened a backed up profile into Thunderbird:

- all previously install plugins (such as ImportExportTools) are wiped out, since the profile's plugins list is activated;

- installing new plugins requires a profile restart, which in my case wipes out the mailbox content, even when working offline at profile startup time;

the combination of those two issues making it impossible to export the mails.

Basically, I have the profile which seems to contain the data, but am unable to access it, due to a combination of factors. Well, I'm pretty sure there must be a way, but it so seems it can't be what you guys kindly suggested so far :)

re :once one has opened a backed up profile into Thunderbird: - all previously install plugins (such as ImportExportTools) are wiped out

You are not following instructions. So I'm assuming you have a complete backup of profile.

Start Thunderbird and install the addons. Go offline How to go offline: click on the two blue screen icon located in bottom Status bar. It will turn into two back screen icon. You are now offline.

The use the importexporttools addon to import the mbox files located in your backup profile into 'Local Folders' mail account. mbox files: see image as guide.

Follow instructions as previously stated. You do not need to open/start Thunderbird using the backup. Open as normal - use the backup to supply you with mbox files which you can import.

options > Advanced > Network and disk space > offline.

Set the option for Thunderbird to start up offline.

Now you get as much time as you need to export, import or whatever before the mail is deleted by a synchronization. Just don't get the idea you can move it etc as you need to be online to do that.