K9 not saving sent mail

[ByteHamster]

I just typed the word “sent” into the search function for you. This is the first result:

Mails sent from phone are not stored in Sent on phone and mail server

I’m using K9 v5.806 on Android 7 (German GUI). The mail server is on my (newly set up) NAS and various mailboxes are synced via IMAP with the Kubuntu Linux PC and Android 7 phone without any problems - at least the inboxes and their subfolders. Sending and receiving messages is no problem on PC and phone. Mail server and PC are synchronized as expected. The Problem: Messages sent from the phone arrive, but are not stored in Sent either in the mail server or on the phone. In K9 in MAILBOX > Sent…