This needs more information. Is this an IMAP account? Are message folders being stored on PC? How are you viewing to see the messages? Website? Are you viewing messages within Thunderbird, or looking at the profile from windows file explorer? With details, I feel this can be fixed.

The messages are from a variety of sources starting with Eudora as the email client. Note I am NOT retrieving the email from a mail server. It's not IMAP, POP, etc. These are complete old archived Email files going back decades.

The file(s) is stored locally on my Linux PC. It was brought over from my backup server (BSD).

All the messages were in a single email file however, Thunderbird started splitting the messages with headers in one file and the body in another. I'm not sure when that started and I can't find a way to put them back together. Thankfully, I have the original RFC-822 compliant files intact.

I place the email file(s) into the appropriate Thunderbird directory and open the local Thunderbird client. All the messages are in the Table of Contents pane. If you click on any message, the body is empty.

If I import the files, I get the same result -- all the messages are in the TOC but the bodies are empty.

Using the standard Unix/Linux mail command, I can view them but, of course, the mime content is just garbage and not rendered.

I hope this helps.

To see if they are, indeed, mbox formatted files for thunderbird, you can view the files for free at https://www.mbox-viewer.com/ that will confirm whether they are legit thunderbird layout. From my experience (and fuzzy memory), messages from Eudora need to be converted to mbox format. That must be done before Thunderbird can properly access them.

That conversion appears to be when Thunderbird, or the conversion separates the headers from the body. For years, I could access the messages using Thunderbird but then, something changed. I'll try some small files and see what it thinks.