There was a point in time ~10 years ago where I used the Gnus email client (emacs). Well, I lost those emails somewhere in my data archive. I managed to find them by grepping my old email address.

At first, it was taking forever, but I discovered the -I option to grep: "Process a binary file as if it did not contain matching data".

Once located, I needed to convert them from NNML to mbox format. I used the script here: https://www.yergler.net/maildir-to-mbox/

Except instead of calling mailbox.Maildir(), I called mailbox.MH().

Finally, all I had to do was import the mbox file with the ImportExportTools NG extension in Thunderbird. My email archive is complete!