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 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?

By default sent messages should be uploaded to the Sent folder. However, this can be disabled. Make sure Settings > [Account] > Sending mail > Upload sent messages is checked.

Thank you for your helpfulness! I had not yet paid attention to this option. However, it is set for all mailboxes. So it can’t be that.

I just tried again to send mails with the phone: they don’t arrive with the one provider ;( From Linux PC (KMail) sending works with all providers. The outgoing mail server settings are as specified by the provider.

I do not know what else to do. Have I perhaps overlooked another setting?

It’s fixed. I deleted the account in question in K-9 and created it again. Now it works out of the box. I don’t know what happened.