Function deltachat::receive_imf::is_probably_private_reply

async fn is_probably_private_reply(
    context: &Context,
    to_ids: &[ContactId],
    from_id: ContactId,
    mime_parser: &MimeMessage,
    parent_chat_id: ChatId
) -> Result<bool>

If this method returns true, the message shall be assigned to the 1:1 chat with the sender. If it returns false, it shall be assigned to the parent chat.