One of the things #Mailpile never did well, was processing bounces and other delivery status notifications (DSNs).
I am currently being side-tracked from what I was working on (sending mail), to add the features #moggie needs to do better.
As a start, I just want to recognize the DSN as part of the thread, the error is a reply to the message. Once the two are linked in this way, I can iterate on how they are presented in the UI.
Mutt doesn't appear to do even this much. Do other mail clients?