Here's an interesting question: How did mail delivery work in the real-world medieval world?
More or less the same way it worked back in Ancient Rome; slowly (compared to current time). Most often the messages were carried by couriers with horses. The requirements to become a courier was to be physically fit (strong and healthy) to survive the journey and know more than one language. In case of religious messages, the courier was required to have knowledge of Latin.
It usually was a freelance work, due the nature and dangers, and the couriers were usually competing against other couriers. It wasn't even rare to have situation where the courier was ambushed by another courier just to get the given task from the original one, especially if the message was important and the clients were wealthy and powerful enough.
I would however believe that those only covered "political" (and i think we can file religious there to) messages.
I could imagine two kinds of letters right now:
- "Political" letters range from stuff like requesting more/less migrants to declarations of war, but gennerally involve the player directly. Those would make sense if they used coutriers.
- "Private" letters would be letters from dwarves to dwarves, like the example with the deceased grandmother. Those only give positive or negative thoughts for the dwarves in question.
private letters should come in via caravan. Perhaps there could be someone to screen letters to sort out the ones that cause bad thoughts, but I think it shouldn't be absolutely shure, or have
some other way of backfiring (Imagine Urist McLovingGrandson finding out his grandma died and apparently no one cared to tell him, or have one of the caravan guys mention to Urist
McYouArentParanoidWhenThey'reReallyAfterYou that his family gave them a letter for him and it never shows up sice it got screened...)
On the other hand I don't know how high the literacy is in DF... Private letters would have been a non issue in our medieval times.