$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
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
$ais, 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 is chiefly intended for use in
configuration-file header definitions. It can also be used in
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