The p macro contains the process ID of the sendmail that executes the delivery agent. Every process (running program) under Unix has a unique identification number associated with it (a process ID). Process IDs are necessary to differentiate one incantation of a program from another. The sendmail program fork(2)s often to perform tasks (such as delivery) while performing other tasks (such as listening for incoming SMTP connections). All copies share the name sendmail; each has a unique process ID number.

$p is intended for use in header definitions but can also be used in the A= equate (A=) of delivery agents.

$p is transient. If it is defined in the configuration file or in the command line, that definition can be ignored by sendmail. Note that a $& prefix is necessary when you reference this macro in rules (that is, use $&p, not $p).