the old postal rule that reflects changes in technology.the precedent is 'if I send it, claiming you never received it, is not a legal defence.'
Well, I hope that rule has been changed. Posted out the Xmas cards in 2015 and many reported "never received", and in fact a number arrived back "return to sender" with no reason given, though we know the address's to be correct. Waiting to see how the 2016 cards fared.