- Delta Chat mode is for users who do not use another “classic” client. When activated, the “DeltaChat” folder is created and used exclusively. “Watch Sent folder” is not available.
- Classic mode is for users who use at least one more “classic” client. When activated, “Watch Sent folder” is available and enabled. “Automatic moves to DeltaChat folder” is available but disabled. Sent messages are stored in the Sent folder of the mail account that is in use.
There are many reasons why users have different mail clients in use. If this applies, Delta Chat should behave like other mail clients, too, when it is about storing messages.
The current situation - messages not stored in the Sent folder when using Delta Chat as a “classic client” - is neither fish nor fowl. It is even worse, because sent messages are currently not stored on the mail server at all when “All” is enabled. The only place where such messages are stored in such cases is the respective client device, so another mail client on another machine can never see them.
Source (adapted/modified): https://github.com/deltachat/deltachat-core-rust/issues/1483#issuecomment-629171282