Recently I found this cover:
The cover was mailed from Dubai to London, England on 11 NOV 1972. It was franked with a 5dh Shaikh Rashid, underpaying both the airmail and surface rates to England.
One of the markings is "Insufficient Postage For / Transmission by Air Mail". The cover also includes three 3p postage due stamps.
Note: I did not see a general postage due forum. Perhaps I am mistaken?
AndrewG*** Moved by Staff to a more appropriate forum. ***