Support proper threading for non-coi users

[sirjofri]

Is your feature request related to a problem? Please describe.

My current test setup is a custom mail server. On my android phone I have your app and DeltaChat installed (with the same server connection). For testing I sent messages to a microsoft exchange server, read mails there via outlook.

Outlook supports a threaded view of mails, which is very helpful when following conversations. I see this as a very important feature for chats. Of course it's a client side feature, but incoming messages to non-coi users are not put in the same thread.

I receive the message from the app and can answer it. When sending another message, outlook displays it as a new mail, starting a completely new thread.

I should note that messages from DeltaChat arrive in the same thread I started before. So there is a possibility to support this.

Describe the solution you'd like

E-mails should be placed in the same thread.

Describe alternatives you've considered

I don't see alternatives.

Additional context

Compare with DeltaChat. It works there. Maybe it's some header field or some envelope thing.

Internal Bug Tracker.