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Headers, Precedence, and Trust
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14.4 Precedence

The precedence of a mail message determines its position among other messages in the queue when the queue is processed. Precedence, as a header line, also defines whether or not a bounced message should be returned to the sender. Precedences are not hard-coded into sendmail. Instead, they need to be declared in the configuration file. The typical declarations, and the ones we will use, look like this:

Pspecial-delivery=100
Pfirst-class=0
Plist=-30
Pbulk=-60
Pjunk=-100

These lines use the P (precedence) configuration command. Like all configuration commands, the P must begin the line. It is followed by a name, an equal sign, and a value. The general form looks like this:

Pname=value

There are only five possibilities for name that are legal: special-delivery This mail message needs to be processed before any others. This precedence is effective only when the message is being delivered from the queue. first-class Unless otherwise declared with a Precedence: header, the message is first-class by default. list The message originated as part of a mailing list. It should be deferred until other more important mail has been processed from the queue. bulk The message is a broadcast, like a mailing list but less important. If the message can't be delivered, the message body is discarded from the bounced mail. junk Absolutely worthless mail. Test messages and mail from some programs fall into this category. Like bulk, the message body is discarded from the bounced mail.

The value assigned to each name is somewhat arbitrary. The ones that we use are common. As you gain familiarity with the queue and mailing lists, you may want to adjust these values. In general, the higher the number, the higher the precedence. By convention, formal first-class mail has a precedence of zero, with positive numbers used for high-precedence mail and negative numbers used for various kinds of bulk mail.

The P configuration command only tells sendmail the value to assign to a given name. It has no other effect. The values are used only when a mail message is processed that has a Precedence: header line in it. The inclusion of Precedence: header lines is left to MUAs.

To illustrate, imagine that a user on your machine is managing a mailing list. The software that is used to create each message for the list arranges to include a Precedence: header that looks like this:

Precedence: list

The mailing-list message is given to sendmail on the local machine. The local sendmail sees the Precedence: header in the message and extracts the field of that header, the list. It then compares list to each of the name parts of its P configuration lines. It finds a match with the line:

Plist=-30

Because it finds a match, it uses the value from this configuration command as the initial precedence of the mail message. If there is no match (or if the original message lacks a Precedence: header), the initial precedence of the mail message defaults to zero.

Now add P configuration commands to the client.cf file. Traditionally, they are placed after the header commands:

H?D?Date: $a                    # Add if F=D
H?M?Message-Id: <$t.$i@$j>      # Add if F=M

# Precedence                                   <- new
Pspecial-delivery=100                          <- new
Pfirst-class=0                                 <- new
Plist=-30                                      <- new
Pbulk=-60                                      <- new
Pjunk=-100                                     <- new


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