When viewing an encapsulated message in the external viewer, it can no longer be printed as a whole. This is because KMPrintCommand expects an Akonadi::Item, but encapsulated messages don't have an item. Also, it is no longer possible to print just the MIME part that is selected in the message structure viewer.
Printing encapsulated messages from a standalone viewer works fine here. I think Andras' recent work has fixed this.