It depends on how you reinstalled. POP messages are in folders on your computer. Copying the entire profile is preferred, but they can also be copied individually, using the OS's file management software to the new computer. In the old computer, they will be in the profile folder and, within that, the Mail folder or the Mail\Local Folders folder.

Hello, and thank you for the feedback! I reinstalled Sonoma on the same computer as it was too old (it is from 2020 and has an Intel processor) to cope with Sequoia. I had to erase everything from the hard drive. I use POP. Despite trying several recommendations from Copilot, ChatGPT and IT people, the problem remains unsolved

You deleted your messages when you erased your drive. Did you make a back-up copy of the data on your drive before you erased it?

Yes, I did

Good! Then you will want to restore your messages from your back-up copy.

Start this way:

1. Make a back-up copy of your drive now.
2. Use the Finder (not Time Machine) to restore the ~/Library/Thunderbird folder from your back-up drive to the desktop (not to ~/Library/). (~ refers to your user account.)
3. Install the add-on ImportExportToolsNG.

Tell us how those steps went before we proceed.

In which folders were your missing messages stored? Were they all in POP account inboxes? Or elsewhere?