You use this to read mails, reply mails, or compose mails to your girlfriend. When there is a new mail, you will see "NEW" icon blinking. If you click Mail command when there's a new mail, you will go directly to read your new mail. The mail options are pretty much like in the First Part of the game (Before Confession) which you can read here. (Note that now you cannot send mails to other girls. They just disappear from your world after you got a girlfriend. :D)
Alert Voice
You can activate new Alert Voice by fulfilling the Monthly Stamp Rally. The commands for Alert Voice are as shown in the picture above. You use the middle button (Listen) to replay the selected voice. If you click confirm, that voice will be your alert voice when new mails are coming.