Potential goal: recipient control of data transfer

Users want fast access to their mail. They do not want to be delayed by the 
downloading of N megabytes of unwanted data (like a load of viruses). This is 
particularly so for those who are still on dial-up access. Those who are 
bandwidth-billed also do not want their expensive bandwidth wasted. They do 
not want other people to be able to mailbomb them or DDoS them with mail.

These goals point to a protocol in which the recipient can be very selective 
about the data transfer process.