Sent folder not syncing properly

My email works perfectly except that messages sent from K9 don’t go to my sent folder. Tried manually syncing, deleted app and reinstalled. I contacted support for my ISP and they had me send a message from the webmail which went to the sent folder. They said this must a problem with the K9 app. How can I fix this? It’s an IMAP account.

Have you checked whether the “Sent” folder in K-9 and on the server are the same?

E.g., I sync my mail with Outlook, Thunderbird and K-9. Thus, I have (automatically created) three different such folders: “Sent”, “Sent items” and “Gesendete Elemente”. Therefore, I need to manually set all three programmes to use the same folder (in my case “Gesendete Elemente” in Thunderbird and K-9, as I use Outlook most frequently).

Hope that helps.

Thanks. How can I check that? In K9 account settings I have ticked the box " upload messages to sent folder after sending"
That’s the only relevant setting I could find.

Go to “Settings” in the sidebar, select your account, select “Folders” and there you can select which folder on the server acts as archive, sent, trash, junk, or drafts folder.