I live in a house. My house has an address. In that house, I have my own room, perhaps a bedroom with en suite.
All the mail that comes into the house, is addressed to a particular person, me being one such.
For reasons that matter to no-one except me, I cannot leave my room.
If I want to send mail to others in the house, I simply write their name on the envelope.
My guardian (who also lives in the house), delivers all mail. Letters come for me regularly - all from people I've written to first mind you. (After all, no-one wants to be the first to start correspondence with a freak). The guardian knows, and implements, this rule tightly. So he brings letters to my room on that understanding. The only people I don't need to write to first, before receiving a letter, are the others in the house, since we are all known to the guardian a priori.
Sadly, I send letters to myself sometimes too. They don't need a house address, only my name, just like the others. (I just give them to the guardian, who hands them right back to me). For good measure, I write my name on the back of the letter, so he knows what to do with it if I am not there. (That was a joke).
One day, just for fun, I decide to spend all my money on a stamp, and write a 'proper' letter to myself. I get someone on the street to post it for me. Next day, it comes back, and the guardian thinks he recognises the handwriting, but he can't be sure, thinks it may be a spoof, and decides to throw it in the bin. I'm left puzzled, because I thought the same rule would apply to that letter as apply to any I send myself in-house. Wrong; 'new' rule. The same rule applies if I try to send a franked letter to someone in my own house.
Making matters slightly worse, if they can be worse, is the censorship of my mail extending to any supernumerary letters which I receive; in other words, if I am in an ongoing written dialog with someone, if they send me even one extra letter, it gets binned, since it doesn't tally with my outgoing missives. This regime quickly impoverishes the nature of my correspondence, making the process of communication pedestrianly and unappetisingly slooooooooooooow. A lot of good information goes into my guardian's bin liner.
On the bright side, those years' of internal scriptural exile pay an ultimate dividend, when I realise I've stumbled across the methodology to allow the invention of the internet.