Recently I had to talk a lot with a person while dealing with an interesting issue on his plot which I support.
He soon went offline and I sent him about 15 emails about the situation development.
It's very uncomfortable - the in-game email has only 1 line, I guess it's shorter than even that Twitter thing

You can say: use TeamSpeak/Forum/E-mail. Well, they all are out-of-game and have other disadvantages:
TeamSpeak is not very popular and it's hard to convince person to install it. It requires being online simultaneously. It is more for people who are real-life friends and come to play together.
Forum is "slow" and it seems not many people are registered. Also it's out of the game, requires switching away, or better disconnect to not die from hunger. Well, PM could be an option, but first i have to ask is the person registered, by which name ...
E-mail - too private, not many people would share emails (I wouldn't ). Also probably not much used now, considering all those modern Facebook-like things.

An interesting in-game offline communication option could be some Book&Quill mail system. Book&Quill is pretty cheap, can be reused many times, allows to write big texts - I'm even trying to make an in-game diary in it.
But how to comfortably pass a book to a recipient, an offline one?

For one-time peer-to-peer thing I could make a lockette-d chest close to Spawn and mail person it's coords.
Comfortable enough, but again takes more time then I would like to spend: selecting place, making chest, putting sign, manually copying coordinates to mail, remembering to destroy the chest ...
I could also make such chest near person's town, but it is even less comfortable - I lack free sethomes and outposts. And some cool people live enormously far.

Is it possible to make some kind of a shared post-office? A one standard place close to Spawn, where it would be very easy to leave such messages to people and exchange them? And also guarantee mail safety.
Not necessarily admin-made or scripted or ender-chest-like. Maybe it is possible to do even by an ordinary user (e.g. me).
Maybe some combination of plot permissions and lockette would allow it easily?
Certainly this raises new questions, e.g. how to manage such mail with potential ~600 towns and ~4000 users ...

Please advice, share ideas, criticize.