The issue you are experiencing could be related to differences between 32-bit and 64-bit versions of Thunderbird. Here are a few troubleshooting steps you can try: Check the file format of the email attachments: Make sure that the email attachments are in a compatible file format that can be read by both the 32-bit and 64-bit versions of Thunderbird. For example, try using the .eml file format instead of .mbox. Check the version of Thunderbird: Make sure that all machines are running the same version of Thunderbird, preferably the latest version. You can check the version number by going to the Help menu and selecting "About Mozilla Thunderbird." Check the location of the email folder: Make sure that the email folder is located in a location that can be accessed by both the 32-bit and 64-bit versions of Thunderbird. For example, try saving the email folder to a shared network drive or cloud storage service that can be accessed by both machines. Try exporting and importing the email folder: Instead of emailing the folder to yourself, try exporting the folder from the 32-bit machine and importing it into the 64-bit machine. To do this, go to the 32-bit machine, right-click on the email folder you want to transfer, select "Export," and save the file to a location that can be accessed by the 64-bit machine. Then, go to the 64-bit machine, open Thunderbird, right-click on the email account where you want to import the folder, select "Import," and select the file you just exported. If none of these steps work, you may need to contact Thunderbird support for further assistance.