Tried to use the mbox view, saw a couple of messages, then just a huge amount of text. Not sure if it was just try to sort out the entire 4 gb of information or if the mbox file was just too big.

I had some questions to help me determine if the mbox file is bad, but they seem unnecessary. If ImportExportToolsNG cannot open the file, it's bad.

Can Syscurve open the file?

Which version of Thunderbird do you have?

Did you get it from thunderbird.net, the Microsoft store, or somewhere else?

David, the option to import from Outlook directly into Thunderbird (if they are on the same computer and Outlook is the default e-mail client) does not exist in 145.0. Do you know anything about its removal? Which recent versions have it?

https://support.mozilla.org/en-US/kb/thunderbird-import

sabata2, do you still have access to your old Windows 10 computer? You could put a Thunderbird version that imports from Outlook on that computer and try the import. I know almost nothing about it.

To answer your questions: Syscurve can see the messages in the file. The version of Thunderbird on my office computer is 145. I have version 143.0.1 on my personal computer. I was successful there in moving my Live Mail messages into Thunderbird. I pulled Thunderbird down from the Thunderbird web site. I still have access to my old computer, and had considered downloading Thunderbird to import from Outlook, but decided to use Syscurve as that seemed a quicker way to move my emails. The first thing I'm going to do today is go to Syscurve and convert one email message and see if I can import it into Thunderbird. If that works I'll try it with a folder, and then move folders one by one. If none of that works, I will go to the Thunderbird Install App I have saved on my personal computer, install it on my old office computer, and see if I can bring over the pst files on Outlook. I'll let you know how that works. Thanks again for taking the time to help, and if you have any other suggestions let me know.

Converted one folder thru Syscurve, brought over to new computer, Thunderbird said "Skip non-mbox file" and nothing happened. Started Mbox viewer, loaded file, and saw all messages (48) in the folder, could read them, and pull down any attachments. If Mbox viewer can take in the mbox file, why won't Thunderbird? Will get on this more later, have to get to other things. PS I'm on the East Coast, so 4 hours ahead of the West Coast.

Converted one folder thru Syscurve, brought over to new computer, Thunderbird said "Skip non-mbox file" and nothing happened. Started Mbox viewer, loaded file, and saw all messages (48) in the folder, could read them, and pull down any attachments. If Mbox viewer can take in the mbox file, why won't Thunderbird? Will get on this more later, have to get to other things. PS I'm on the East Coast, so 4 hours ahead of the West Coast.

Just to be thorough: You referred to "Thunderbird" twice. Did you use ImportExportToolsNG?

Some people have reported that ImportExportToolsNG cannot handle a file larger than 4 GB. You could make a copy of that file and use a text editor to split it in two, then try to import one of the two files.

The developer of the add-on discussed the size issue here: https://github.com/thunderbird/import-export-tools-ng/issues/323 and here: https://github.com/thunderbird/import-export-tools-ng/issues/452

You could ask for help on his support forum: https://github.com/thunderbird/import-export-tools-ng/issues

Time zones: support volunteers are spread over the planet.

I used the ImportExportToolsNG to try to bring in one folder converted to MBOX from the PST file. The file's size was 80 mb with 48 messages, all were visible in Mbox Viewer but not in Thunderbird (v145). I tried to read the one 80 mb Mbox file on the earlier version of Thunderbird (143) on my personal computer, the Mbox file was not recognized (i.e. got the "Skip non-mbox files" message). Therefore I don't think size is the problem here. I'm going to ask Syscurve Support if they can explain why their conversion does not work on Thunderbird (v145). I notice that in the ImportExportToolNG on the Thunderbird (v143) on my personal computer I could import EML files, but not on the latest ImportExportToolNG. The Syscurve App can create EML files, MSG files, and HTML files as well as MBOX files. I'll follow up with your suggestion to go to the Support Forum for the ImportExportToolsNG. I feel like I'm just missing something in this conversion process. Thanks for the help.

I notice that in the ImportExportToolNG on the Thunderbird (v143) on my personal computer I could import EML files, but not on the latest ImportExportToolNG. The Syscurve App can create EML files, MSG files, and HTML files as well as MBOX files.

The menu option to import eml messages appears only if the user right clicks on a folder, not an account name. The apparent logic is that individual messages need a folder to be imported into, whereas mbox files need only an account because they are, in effect, a folder of messages.

Trying eml is a good idea.

I was able to convert the PST files to EML, and then use the ImportExportToolNG to bring in the file structure and the email messages with attachments. I've reached out to Syscurve on the issue that the ImportExportToolNG gives me the message "Skip non-mbox file." I now have the old emails in Thunderbird so that problem is solved. If Syscurve gives me a solution to the MBOX file problem I'll post it here. Thanks for all the help I've received, and I'm glad to be moving on from Outlook. Best to all.

Good news! Well done.