no. 4: email airmail

Often, when communicating with people, you don't want an instant response, or for them to get the message right away. For instance, when keeping in touch with an old friend, the immediacy of email creates an odd impetus for rapid and continual dialogue. Create a service that simulates the (somewhat random) delay brought on by normal mail. So send a message to your friend, and, depending on their geographic distance, it delivers it in around X days. They respond, and you can have more of a correspondence with less pressure...