Why do we have incidents with threads that say "Unsupported message type - delivery-status, message attachment discarded."
Outbound / Feedback
Threads that have the text "Unsupported message type" are created from bounced messages.
By default, bounced messages do not create threads however mailbox configuration can be changed to not filter the messages. Unless you have a specific business case for processing bounced messages, it is suggested that you leave the Delete Returned Messages setting checked.
1. Open the mailbox configuration under Configuration -> Mailboxes -> select the mailbox, click Incoming Mail in the ribbon.
2. Check the box beside Delete Returned Messages.
3. Save and close the mailbox.