A while ago I wrote about barring junk mailers and doorstep pests using a gate that can be unlocked by contacting the householder (via a dedicated phone number). Still longer ago, in 2009, I wrote about a company that sold a letterbox lock which could be opened only by the postman (who would be given a key). If such anti-junk mail strategies sound appealing you'll be interested in a system invented by Simon Page:
What about a keypad on your letterbox where the postman, TNT or other deliverer, enters the second part of your postcode to open the letterbox? The pizza boys don't know it but the Post Office and couriers have it written on the letter / parcel!
I reckon it's a smart idea. The company that produced the letterbox lock I wrote about in 2009 went bust quickly, presumably because Royal Mail didn't like the idea of postmen having to take a bunch of keys with them. A letterbox that can be opened by entering the outward part of the postcode does away with that problem. Similarly, an issue with the gate lock would be that deliverers who don't know the code to unlock the gate would have to contact the recipient – that's a bit of a hassle and might not go down well with genuine deliverers.
The only potential flaw I can see is that the new, reinvented lock-system wouldn't stop radical junk mail delivers; the ones that will ignore anti-junk mail signs because they're convinced that their junk mail isn't junk. They could simply find the postcode on their smart-phone and unlock the letterbox. But then such junk mail jihadies would by-pass any system. My milkman, for instance, respects the 'No Junk Mail' sign on my door but will leave the occasional leaflet on my doorstep.
It would deter lazy junk mail deliverers though, as well as those who don't have enough command of the English language to understand the meaning of 'No Junk Mail' – they wouldn't understand the instructions to unlock the letterbox either.
Anyway, at the moment the lock exists on paper only. Please contact me if you'd like to develop it – I'll put you in touch with its inventor.