Workaround
I realized that there is a workaround and right now I can’t see any problems with it. What I did to import the old messages lying around in the DeltaChat directory in my mail was this:
- Create a directory and call it anything, like “e-mail parking”.
- Move all your mails in your inbox over to that directory.
- Move all your mails in the DeltaChat directory into your inbox.
- Delta Chat (on Android and Desktop at least) notices these messages and they appear in the client.
- Move back these messages to the DeltaChat directory.
- Move back your inbox messages from the “e-mail parking”-directory to your inbox.
Tada