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.