Mail stores rewards from daily logins, guild tasks, purchases, special events, refunds, and any other distributions in the game.
Mail can be accessed from the World Map via the Mail button on the right side of the screen.
Mail can be viewed individually by pressing the View button on a specific item. Each item can be claimed by pressing the Claim button therein, or by pressing the Claim All button at the top of the screen.
It is worth noting that each mail item has an expiry date - typically a week (for things like guild tasks or weekly rewards) but daily rewards expire in 24 hours. Collect mail before it expires!