How many messages in these folders you wish to upload to the IMAP server? I'm not sure of the exact parameters, but a large number will involve a significant bandwidth load, for which the IMAP system is not designed, let alone whatever restrictions may be imposed by the mail provider. That probably explains why the add-on doesn't allow it and drag and drop of folders doesn't work.

If you really need the messages on other computers, I'm afraid manual drag and drop of small numbers at a time is the only practical way. Or, copy the mbox files from Mail/Local Folders in the profile of one computer to the same location on the other computers. Crude, but effective, and works with large files, either manually or over a network.

Thank you, your reply cleared up some confusion and gave me a possible explanation to why things weren't going so well.

The IMAP server is a Gmail account. I have already upgraded Google One to have plenty of server space for what I wanted to do. Bandwidth, of course, is another subject.

There are large numbers of emails; hundreds in some of the folders, though the majority are closer to "dozens." And there are a couple of hundred folders involved.

I was of course experimenting only with a handful of them, just to see if I could do what I wanted to do. I'm guessing the answer is "no," simply due to the volume.

The folders contain saved emails I access on a daily basis. I was trying to find a way to keep them always up to date on two different computers (a Mac desktop and a PC laptop ... the latter is actually the primary email computer, but I frequently have need to check in from the other system as well).

I'll give simply copying the local folders into the Mac's profile a try. It would mean manually repeating the process on a daily basis, to keep things relatively in sync, but it's doable.

Thank you for the suggestion!

I think if you move the existing folders manually to the other computer, and then store new saved emails in a separate IMAP folder, it should only be necessary to move that folder to Local Folders, on both computers, when it gets too big. In the meantime, new saved emails are accessible form both computers, as well as any that are already in Local Folders.

Thank you for the additional suggestion. Since there are several hundred folders involved, and the topics/recipients/senders need to be kept separate, I'm not sure how I would juggle that possibility, but I will certainly think about it as a possible solution.

I greatly appreciate your time!

The issue is that the IMAP protocol has a move message, but it does not have a move folder. https://tools.ietf.org/html/rfc6851

This is compounded by Google not having a full RFC compliant IMAP implementation as they have no concept of folders at all, using internal flags on messages in all mail to create pseudo folders which they display on the web and offer up to IMAP mail clients.

I've noticed that Gmail has this "labels" thing, and realized yesterday that what I see in the web interface as "folders" are actually just false folders created by certain labels. This, of course, means that Gmail is lumping all of those messages (which would have numbered in the thousands if I'd tried to do what I wish I could do) into one inbox and labeling them as being in pseudo-folders.

Your explanation confirms what I was observing, thank you.

Nothing's simple.

I appreciate everyone's help. Thanks!