Let's not confuse protocol with lawful requirements. Yes rank is important, structured authority is important, and it all ties into respect - which is sadly lacking in the civilian world - but how far do you want to take it. I can't believe the 'signature block creep' that I see with the civvies and ex-military. Some people include every degree they have, it started with a few doctors who had their PhD, and then spread to the professional engineers, and then the ex military folks (MMM, CD, etc, etc) to everyone with a basic degree including the full list after their signature, on the most informal inter-office correspondence. Give me a break. I've got a few that the GG says I can use after my name, but I'll save it for letters to my MP, or other such 'formal' matters. Same as the rank (outside of military circles).
I'm sure that many on these boards remember the wives wearing their husbands rank. Who wants to see that in wives/husbands today - to the extent that it used to be.
As for the example of returning mail that doesn't include rank - good way to make a point, and I don't fault that - so long as there was nothing that was actually time-sensitive that got 'returned to sender' because you wanted to make a point to the civilian originator.