P1: protocol used for the communication from MTA to MTA. It can be compared with the address writen on the envelope containing your letter and which allows to carry out the sorting and the routing of the mail.

P3: access protocol to the MTS. Used for the communication enters MTA and MS or between MTA and an UA.

P2:  protocol of interpersonal electronic mail. Used between two UA, it can be compared to the standards of letters presentation.

P7: protocol used between MTA and an UA who is not directly connected to a MTA. In fact it is very similar to P3, except a service of storage intervenes (using Message Store) - apart from the MTA and the UA.