SMTP Service Extension for Priority Message Handling
This memo defines an extension to the SMTP (Simple Mail Transfer Protocol) service whereby messages are sent with a priority to achieve a certain order in which the messages are transferred by an MTA (Message Transfer Agent). This priority or precedence order is used instead of the first-come-first-serve rule. This extension is not to be confused with "Importance of a Message" which is widely deployed using an email header such as Importance or even Priority or Precedence with common values of HIGH, NORMAL and LOW. Importance of a Message does not affect the priority of the transport itself in any way. Nevertheless, there may be policy defined relations between priorities and importance indicators. This extension uses the term priority in the meaning of expedited treatment of a message by the server according to its priority.