The Full-Name: header is used to list the sender's full name if it is known. The field for this header can be arbitrary text but is usually the value in the $x macro ($x):

H?x?Full-Name: $x
H?x?Full-Name: (User names hidden for security)

The Full-Name: header should be prefixed with the ?x? flag so that selected delivery agents can require inclusion of that header. This header definition is usually meaningless because all traditional delivery agents omit the F=x flag. It can be useful, however, for mailing through sites that strip or destroy From: headers.

Full-Name: was originally designed for UUCP use. Early versions of UUCP could not accept full names in From: header lines:

From: host!user ( full name )  did not work for early UUCP

The Full-Name: header can be specified in the configuration file. If this header is already in the mail message, sendmail does not replace it.