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.
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21.9 Alphabetized sendmail Macros | 803
$a is transient. If defined in the configuration file or in the command line, that definition
might be ignored by sendmail.
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