yes, i'm pretty sure the heart in the mail boosts your tama's happyness level.
And also, here are the explenations of the icons at the post:
Star: It's your fortune. POINTS ICON= If you have 3 stars show up, that means there is a good chance you will get more points when you play a game. 2 stars= normal points 1 star= less than normal points.
BODY: It tells you if your tama is going to get sick, 1 star= a likley chance it'll get sick, 2 stars= normal chance 3 stars= no chance of getting sick. HEART: I think this represents if your tama will get along with the next tama it connects to. 1 star= it'll hate the tama, 2 stars= it's an equal chance, 3 stars= Your tama will be great friends with the connecting tama!
envilope: You can get all sorts of things in the mail, like the heart you got, you might even get something from the king, like a present or points. But you can also get poop and a snake in the mail. You can even get robbed!
[!]: When this icon is flashing, that means you are eather going to attend school, or try out for a job... or you are going to graduate from the school.
hope someone can help! 
