That's awesome. I wish more games would come up with creative ways to integrate "IRL" into their in-game environment.

This reminds me of the Wii U port of Legend of Zelda: Wind Waker. During the Wii U/3DS era nintendo had their own social network called miiverse, which was functionally identical to twitter but games could access and post messages to your miiverse account. Wind Waker's integration was to let you scribble down a miiverse post onto a sheet of paper (via the Wii U's touchscreen) and Link would roll it up and shove it into a bottle then throw it out to sea. It would then wash up on a beach in somebody else's game and you likewise would find other peoples' miiverse-in-a-bottle posts scattered all over the beaches of hyrule.

The Civilization series absolutely needs mail integration..

"Are you sure you want to play one more turn? You have 8,371 unread messages."

(Full disclosure: That's the current unread message count for my wife's inbox, NOT mine ;) )