Not all messages need to be treated equally. Messages that have failed often, for example, tend to continue to fail. When sendmail processes the messages in its queue, it sorts them by priority and attempts to deliver those with the lowest priority value first.

When a mail message is first placed into the queue, it is given an initial priority calculated when it was first created (RecipientFactor), which is stored in the P line:

P640561

This number in the qf file is really a cost. The lower it is, the more preferentially the message is treated by sendmail. Each time the qf file is read, the number in the P line is incremented. The size of that increment is set by the value of the RetryFactor option (RetryFactor). If that option is negative, this logic is inverted.

The form of the qf file P line is:

Ppri

The P must begin the line. The pri is a text representation of an integer value. The pri must immediately follow the P with no intervening space. The text in pri is converted to an integer using the C-library routine atol(3). That routine allows pri to be represented in text as a signed decimal number, an octal number, or a hexadecimal number.

If pri is absent, the priority value used is that of the configuration file RetryFactor option. If the entire P line is absent, the priority value begins as zero.

There should be only one P line in any qf file. Multiple P lines cause all but the last to be ignored.