From Official Wiki of the Renaissance Kingdoms

You can access your mailbox by clicking My letters in the main menu.

There, you will be able to see:

  • Letters received;
  • Letters sent; and
  • Letters archived

You can only access your archived letters if you are a Noble of the Robe or a Noble of the Sword. This premium packs will also let you: see if your sent letters were opened or are still unread, and to add a personalized signature to your letters. You loose this features once your pack expires, but in the case of the archived letters, you'll only loose access to them, up until you reactivate any of the nobility packs available.

To write a letter to someone, you must either "Enter an addressee" or to select one from your contacts list.

It is mandatory to add a Subject to your letter, and you have a limit of letters that can be sent daily before you are also required to add a captcha before sending your letter.