The $a macro holds the origin date of a mail message (the date and time that the original message was sent). It holds a date in ARPAnet format, defined in RFC2822, section 3.3.

The sendmail program obtains that date in one of the following four ways:

• When sendmail first begins to run, it presets several date-oriented macros internally to the current date and time. Among those are the macros $t, $d, $b, and $a.

• Whenever sendmail collects information from the stored header of a message (whether after message collection, during processing of the queue, or when saving to the queue), it sets the value of $a. If a Posted-Date: header exists, the date from that line is used. Otherwise, if a Date: header exists, that date is used. Note that no check is made by sendmail to ensure that the date in $a is, indeed, in RFC2822 format. Of necessity it must trust that the originating program has adhered to that standard.

• When sendmail notifies the user of an error, it takes the origin date from $b (the current date in RFC2822 format) and places that value into $a.

$a is chiefly intended for use in configuration-file header definitions. It can also be used in delivery agent A= equates (argument vectors), although it is of little value in that case.

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