What are P-Series Protocols?
P-Series Protocols are a group of protocols that are part of the X.400 messaging standards. Five P-series protocols relate to messaging systems that support X.400, such as Microsoft Exchange Server:
P1 Protocol specifies the layout of messages transferred from one Message Transfer Agent (MTA) to another. This protocol specifies that X.400 messages consist of two parts: a P1 header, which acts as an envelope and must contain a globally unique recipient address for message routing and control purposes, and a P2 message, which is the actual content of the message.
P2 Protocol defines the format for transmitting the content of an X.400 message. This format includes a P2 header (which is not used because the P1 header provides the necessary routing information for the message) and a P2 body, which is the actual content of the message and consists of one or more body parts of various types (such as text, images, voice, or telex).
P3 Protocol specifies how a user agent (UA) communicates directly with an MTA for sending or receiving a message. This protocol is not used as often as the P7 Protocol for the same reason that Post Office Protocol version 3 (POP3) is used instead of Simple Mail Transfer Protocol (SMTP) to receive Internet mail.
P7 Protocol specifies how a UA communicates with a message store (MS) in order to selectively retrieve messages from the store and delete unwanted messages without downloading them.
P22 Protocol is a 1988 revision of the P2 Protocol that clarifies and extends certain features of P2.