On Monday, 31 July 2023 17:25:20 BST Peter Humphrey wrote:
> On Monday, 31 July 2023 17:03:49 BST Peter Humphrey wrote:
> > I already have it set up, so I hope I'd only have to deselect "Download
> > all
> > messages for offline use" and then drag the locally stored emails to the
> > IMAP Account, which is shown at the top of the folder list, attached.
> > Does that create a copy of the local directory structure?
> 
> Hah! I just tried the folder-move function in KMail, and it crashed. 79
> emails and 2.2MB.

I have used Kmail to download large-ish IMAP folders (4000+ messages and 
~800MB) to local folders, then upload them to different remote IMAP account/
folder.

It has worked reliably here and ought to work in your case too, if:

- Your Internet connectivity is stable and the bandwidth is > than dial up, or 
you work locally with dovecot.

- You create the local/remote Kmail folders first and then copy messages over 
in small numbers.  About 50-100 at a time should do it, depending on size of 
attachments.

- You remain patient until the messages have downloaded, but then you really 
remain patient until akonadi finishes indexing them.  This may take longer 
than you wish.[1]

- Do not start with another folder, until the current folder migration has 
completed successfully and you can verify the content of at least a sample of 
messages has landed where you expect it to be.

[1] Sometimes synchronising a large number of messages/folders will appear to 
be stuck, because Kmail won't display it.  A workaround I use is to close 
Kmail and run in a terminal:

akonadictl fsck
akonadictl vacuum

waiting for each command to finish, before I restart Kmail.  This is not a 
regular occurrence, but when Kmail misbehaves the above forces a sync so the 
next batch of emails can be copied over.

HTH.