I think that as you have Delta Chat only running on one device, it’s better to disable Advanced :: Send Copy to Self as a temporary solution so that there are no sync messages at all. And keep Move automatically to DeltaChat Folder (mvbox_move) disabled as you’re using other MUA. But note that your server should copy outgoing messages to some Sent folder, otherwise you won’t see messages sent from Delta Chat in other MUA. It seems that the proper fix is to always move sync messages to the DeltaChat folder, even if mvbox_move is disabled, except the chatmail case where the DeltaChat folder isn’t needed.