Perhaps not much, but last year I was looking at using IMAP as an export/import path for a relative's old Eudora mails to some much-more-recent client. I insisted on it, after I found out it could no longer make a secure connection to their e-mail server.
That said, I was saved/pleased by Eudora2Unix [0], one of those projects that represents a very long slow burn of successive people with a similar niche struggle. You might think Thunderbird would have an import tool for that, but it's been so many years it didn't survive...
Out of curiosity, did you have much luck using Eudora's IMAP to do the export?
Yes, actually. I could (in Eudora) add an IMAP account and drag individual messages into it, and then later launch Thunderbird to pull from the same account. I am less-certain about issues around nested folders, which seem to be a magnet for oddities when it comes to e-mail clients and servers.
One of the reasons I kept looking for other approaches is due to how Eudora handles attachments. In this case, many of them had been "detached" from the message they came from. In other words, the message bodies no longer contained the data, but instead a placeholder pointing to a real file in a massive Attachments folder. (Not sure exactly how it handled name collisions.) Uploading such a message to IMAP would leave the attachment(s) behind.
The linked version of Eudora2Unix (unlike the Sourceforge version) tries to reverse this process and reconstruct the original messages, encoding and inserting attachments back into the body. It doesn't always work, but I suspect some of the failures I see are because the file content was deleted over the years by the user to free up disk space, without removing the e-mail trace.
Another plus is that I could just grab all the files for the user and then promise to come back with something better, as opposed to babysitting the upload/download process.
Thanks for sharing your experience and insights on this. Migrating out of Eudora is no walk in the park! I've often considered taking the IMAP route, but Eudora's support for IMAP feels a little patchy.
Specialised conversion/migration software is probably the better route to go, especially now that the Eudora source code was donated to the Computer History Museum. I'll give Eudora2Unix a shot though!
You mentioned attachment name clashes: Eudora simply appends an incrementing integer to the incoming attachment's filename, for example, it would retain file.jpg for the first email and use file1.jpg in the next email it encounters.
FWIW it wasn't took hard to make a little Dockerfile with python:2.7-alpine and have it bind-mount the a directory of data and run in there. (With a USER directive setting it to my UID.)
Note that its "pine" output format is... almost what Thunderbird wants, except for nested mailbox effects. That'll require renaming directories from foo to foo.sbd and sometimes adding a corresponding blank foo file.