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 only the messages stored on the mail server (and PC) are displayed. I’m especially irritated that only one mailbox has these problems and I can’t find any differences in the settings.
In K9, under Settings > MAILBOX > Folder > Sent Messages folder: Sent is specified.
Under Settings > MAILBOX > Retrieve emails is Folders to retrieve: main folders only and Push folders: none specified (retrieval frequency 15 min is enough for me).
Under Mailbox > Manage Folders > Sent is Display class: none, Query class: same as Display class, Push class: secondary folders, Notification class: same as Push class specified.
How can I save the messages sent from the phone in Sent on the phone and the mail server?