The Posted-Date: header
is used by some old Usenet news software and some mailing-list
software to indicate the date and time that a mail message was posted
(submitted for distribution). The Date: header, on
the other hand, shows when the message was mailed. In actual
practice, the two usually show the same date and time.
When sendmail tries to determine the originating
date of a mail message, it first looks for a
Posted-Date: header. If one is found, it uses that
date. Otherwise, it uses the date from the Date:
header. Whichever is used, the result is stored into the
$a macro ($a).
The Posted-Date: header is not a part of the
RFC2822 standard, so it should not be declared in the
sendmail configuration file.