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
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
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
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
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
P22 Protocol is a 1988 revision of the P2 Protocol that clarifies and extends certain features of P2.
