Clicking on it will lead you to your inbox. Keep in mind there is a maximum amount of messages that can be stored in your inbox, depending on your donating status.
If you want to send a message, click Compose, all the messages you have sent, can be found in your Outbox.

If you want to send a mail for more than one recipient; Go to compose, and where you're supposed to place the user nick, just place the nicks you want separated by ‘;’ (Ex: user1;user2).
You can only do this for a maximum of 4 users.

Have in mind you can only send up to 2048 characters in a message.

When you receive a mail with a white cross in front, the mail is sent to your previous account, for example you can receive a testament.
But all the money and cars which are mentioned will NOT be added to your account, since it was meant for your previous account.

When you are a capo, or top 3 in your family, you can send a mail to all users in the family, all capo's, and all who are in your capo regime.