I have the mail folder of an Opera (12, Linux) installation long gone, with 3.6GiB of e-mail data. Even Opera Mail is not available anymore, so I don't think I have a chance to directly open those e-mails. Many of them are also not on the server anymore, and two of the e-mail accounts have since been terminated.
I'm trying to set up Thunderbird, on openSUSE Tumbleweed, to use those e-mail accounts and give me access to my old mails. Usually Thunderbird offers to import mails from other installed e-mail software but as I said, Opera is not on this system, and I don't seem to be able to get hold of Opera Mail anymore. This rules out the use of the most popular method of moving mail over from Opera, which includes exporting the mail from Opera first (should have known before I nuked that installation...)
I have also found the ImportExportTools extension for Thunderbird, but I tested it on a very small subset of the messages, and apparently it generates a new folder for every .mbs file in the Opera mail store. This is pretty awful since Opera switched to storing every single message in a separate file at some point, and one of the accounts has about 20k e-mails in it...
I guess I could write a script which concatenates all mbox files for a single account, and then import that, but I was wondering if I'm overlooking a better way. Maybe there's even something which would let me keep the folders/views I had set up in Opera (my goodness, were those things handy!)?
Any ideas? I'd be okay to move to a different email client if that makes things easier.