The Date: header specifies the date and time that the mail message was originally sent. All mail messages must include this header line. Consequently, the Date: header must be declared in the configuration file like this:

H?D?Date: $a

The $a macro ($a) is mandatory in the field for this header. The value in $a is the current time in RFC2822 format. (See Section 5.1 in RFC2822 and Section 5.2.14 in RFC1123.) Only the $a macro should be used with the Date: header because it is the only one that is guaranteed to contain the current date and time in RFC2822 (and RFC1123) format.

The ?D? flag is always included with the Date: declaration in the configuration file. All the standard delivery agents always include an F=D flag (F=D). The ?D? allows custom delivery agents to be designed that do not need a Date: header.