Kmail does not support INBOX.

[dragon at peerfreedom.org]

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.