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Sendmail, 3rd Edition by Bryan Costales

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Precedence

The cost of a mail message determines its ability to be sent despite a high machine load (and its position in the queue depending on the setting of the QueueSortOrder option, QueueSortOrder). Each mail message has a precedence and a cost. The initial precedence (sometimes called class) of a mail message is defined by the optional presence of a Precedence: header line inside the message with a symbol corresponding to a value defined by the P configuration command.

For example, if your sendmail.cf file contained this line:

Pspecial-delivery=100

and your mail message header contained this line:

Precedence: special-delivery

your mail message would begin its life with a precedence class of 100. We’ll cover how this is done soon.

After the message’s initial class value is set, that value is never changed. As soon as the class is determined, the initial cost is calculated. This cost is the value that is used to determine if a message will be sent despite a high machine load (defined by the RefuseLA option, RefuseLA, and the QueueLA option, QueueLA) and to determine its order in queue processing. The formula for the initial calculation is the following:

cost = nbytes - (class * z) + (recipients * y)

where nbytes is the total size in bytes of the message, recipients is the number of recipients specified in the To:, Cc:, and Bcc: header lines (after alias expansion), and z and y are the values of the ClassFactor option (RefuseLA) and the RecipientFactor option (QueueLA).

The Precedence: ...

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