So Petmail is designed to bring computer communications back to the standard
 that exists for traditional human conversation. If I don't know you, and you
 want to consume some of my time, you have to be willing to spend some of your
 own. The "you" in this context is the *human* who is trying to contact you.
 They don't get to use a machine to help them consume your time.
 
 The difference between spam and mail from new people is that spammers don't
 want to initiate a conversation and are unwilling to personally dedicate
 themselves to each and every recipient they are trying to reach. They use
 lots of computer time (frequently stolen these days) to send unidirectional
 messages. If they had to be involved in each one of those messages, they
 would become just another random person out on the net who saw your address
 and wanted to say hi.
 
 [rate limiting]

    [old]
    So the goal is to set a new expectation for how computers are used in
    human communications, in which we remember that there is a person at the
    other end of the wire. We make email more like human communications...
   
    sending end: introducing yourself at a party, or over the phone: we are
    very conscious of the time we're consuming, and we usually start with a
    justification of why they should listen to us.
   
    receiving end: we don't blow off someone who takes the time to say
    hello. If someone stopped you on the street to ask a question, or called
    you on the phone, we give them a few seconds to make their pitch for our
    time because our experience tells us that it may turn out well, and we
    may make a new friend or establish a connection. One of the most bitter
    consequences of the current spam epidemic is that it discourages this
    openness.
   
    When someone puts their own time into the communication, it forms a
    natural rate-limiting mechanism. Their time is valuable too, they have
    their own 2.5Gs to allocate. Think about the difference between a phone
    call from a stranger and automated telemarketing robots: the reason that
    the automated process is so much more annoying is because the sender is
    not a human who feels the loss of those seconds from their life: the
    telemarketing process is not putting any human time into the call (to
    save costs), which removes the natural limiting.
   
    So when making new connections we impose a rule: if you want to consume
    some of my (human) time, you have to be willing to spend some of your
    own. This brings the process back in line with humans talking to each
    other.
   
    We allow different rules for friends, since in the real world we give
    more access to people we trust (home phone numbers, indulge late-night
    phone calls).