$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
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
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