1) Throughout your journey of developing a thriving civilization you will form relationships (both good and bad) that will affect your gameplay. Part of the mechanics related to these relationships are found within the mailing system. You have a chance of receiving mail from NPCs (currently either gift mail or quest mail). The odds of receiving mail from NPCs depends upon your reputation with these characters. 

2) Gift mail comprises a set of gifts that a character may send you from time to time depending on your reputation with them.

3) Quest mail can be sent from NPCs with an assortment quests (more quests will be added over time, currently King Elrich has the first established quest sent from an NPC). These quests when completed will improve your fame score, and your reputation with the character, and from time to time will also include valuable rewards.