Should that happen, you can probably make corrections. Please share details on exactly how you did the transfer and we can go from there. Thank you.

details look like http://mzl.la/1ApHiU3 deepL-Transation: Method using a data transfer device On your source computer Connect a data transfer device to your source computer. This can be a local device (e.g., a USB flash drive) or an external storage medium. Click the menu button, click Help, and select Troubleshooting Information. In the tab that opens, go to the “General Information” section, find the entry Profile Folder, and click on the Open Folder button. Your Thunderbird profile folder will now open in Windows Explorer. Close Thunderbird. In Windows Explorer, go up three levels in your Thunderbird profile folder. Right-click on the Thunderbird folder. Then select Copy. Right-click on the device you are using for data transfer. Then select Paste. If you are using a local device (e.g., a USB stick) for data transfer, safely remove this device from the source computer. Tip: You can also use these steps to create a backup copy of your Thunderbird profile folder.

On your destination computer Connect the same device you used to copy your Thunderbird profile folder from the source computer to your destination computer. Right-click the copy of the Thunderbird profile folder you created earlier on the data transfer device. Then select Copy. Open Thunderbird. (If the window for setting up email accounts appears, close it.) Click the menu button, click Help, and select Troubleshooting Information. In the tab that opens, go to the Profile Folder entry in the General Information section and click the Open Folder button. Your Thunderbird profile folder will now open in Windows Explorer. Close Thunderbird. In Windows Explorer, go up three levels in your Thunderbird profile folder. Right-click in the folder. Then select Paste. When prompted, replace your current data. Alternative method for manual transfer between computers running Windows 10 and Windows 11 The instructions in this chapter are based on you having a standard installation of Windows and using computers that have been upgraded from earlier versions of Windows to Windows 10.

Locate the main profile folder on your source computer (as described above). Locate the main profile folder on your destination computer (as described above). Keep in mind that your username (identified by the ~ character, see the file paths below) may be different on the two computers, but this is irrelevant for the manual transfer described here. Completely exit Thunderbird on both computers. On your source computer, copy the Thunderbird folders contained in the two file paths Windows(C:)/Users/~/AppData/Roaming and Windows(C:)/Users/~/Appdata/Local as separate folders to a data transfer device (USB stick, external drive, or similar) so that they are not overwritten. On your destination computer, delete all Thunderbird folders in the destination folders Windows(C:)/Users/~/AppData/Roaming and Windows(C:)/Users/~/Appdata/Local, or rename both Thunderbird folders if you need a data backup or want to keep your old profile. Copy the correct Thunderbird folders that you previously saved separately from your data transfer device to the two target folders mentioned above on your target computer to replace the deleted or renamed Thunderbird folders. Start Thunderbird on the destination computer and test it. Make sure that the inbox tab is identical to the tab on the source computer, but other tabs with Thunderbird messages may have been opened.

details look like http://mzl.la/1ApHiU3

The original English version is https://support.mozilla.org/en-US/kb/moving-thunderbird-data-to-a-new-computer

OK now tell us what is incomplete. what error message, if any, are you seeing?

Unfortunately, after copying the Thunderbird-profile to the new PC, only about 30% of the original email subfolders and emails are available. Many important email folders that I created myself and their contents are missing!

On your old PC had you moved certain parts of your profil folder (message files, e.g. Local Folders) to another directory outside the profile folder?

Look in the exported profile to see if you see the missing folders: - the imapmail\<IMAPalccount> folder - the mail\<POPaccount> folder - the mail\Local Folders folder for other folders

If you see them there and they are not in the active profile, you can simply copy them when thunderbird is not running.

Thank you very much for the advice and comments. I will try this out tomorrow with the client I volunteer for. I will then report back here. Best regards and thanks to David and Mapenzi.