It's signed, sealed and delivered (4)
(Other definitions for mail that I've seen before include "Flexible armour" , "Dispatch - armour" , "Letters and parcels" , "Armour; posted items" , "American post from Lima" .)
(Other definitions for diploma that I've seen before include "official document" , "Document conferring a qualification" , "Course certificate" , "Certificate awarded by college for educational attainment" , "degree certificate" .)