Tamagotchi v4.5 mail icon meanings

I hope I'm not posting too much. It's not gonna keep happening though I have looked after an answer for this small question, but couldn't find any.

This will probably be my last question for a long while (I hope).

So the mail icons for v4.5.. I don't understand what they mean. I understand the fortune, and the points, but the heart icon, and the skull icon ? And I think one more.. I got the heart icon. It must be something good, I figured, since my tamagotchi jumped with joy But yeah.. anyone?

It's a love letter ^_^