sdb files are not files. They are system folders/ directories.

Using the inbox as an example.

Inbox.msf This file is the index (sometimes called the database) it basically populates the internal message lists and contains references to the actual email locations (offsets) in the inbox file.

Inbox This is the inbox mbox file. Thunderbird uses no file extensions for mbox files and if they are placed in the profile folder with them they will appear in the folder list in Thunderbird with them.

Inbox.sdb This is the system folder used by Thunderbird to hold the mbox files for sub folders of the inbox folder.

I have some images in my blog post from probably 10 years ago https://thunderbirdtweaks.blogspot.com/p/blog-page.html that might help explain.

Hi Matt,

thanks for your comments!

Having experimented with the some of my *.mbox files, I now understand that *.sbd files are required to indicate the hierarchical structure of the *.mbox files to Thunderbird. On a small scale I could replicate the Outlook hierarchy by creating *.SBD files in the required locations.

Unfortunately I have about so many *.mbox files that creating the *.sbd files manually is not really an option.

It is a shame that the 'ImportExportTools NG' flattens the existing hierarchy of the *.mbox files in the Windows file directories (see attachment) and turns these into a flat structure in Thunderbird.

Best regards,

Volker