kmail fails to support the INBOX of IMAP servers.
If you add a new email account, then you will not see any emails in Inbox.
The folder called "Inbox" in the folders tree, is visible, but empty.
You need to enter account settings, there go to folder subscription, and
then you will see folder called "INBOX" (in caps), and it does not have
the checkbox selected.
Select that checkbox and download emails - now they will appear in the
Inbox folder.
"kmail 4:18.08.3-1" but kmail about says "Version 5.9.3"
using god-damned Akondai with MySQL backend
from Debian 10.
