mail-header-format

mail-header-format is a compiled Lisp function in `mailheader.el'.

(mail-header-format FORMAT-RULES HEADERS)

Use FORMAT-RULES to format HEADERS and insert into current buffer.
HEADERS should be an alist of the form (HEADER . VALUE),
where HEADER is a header field name (a symbol or a string),
and VALUE is the contents for that header field.

FORMAT-RULES is an alist of elements (HEADER . FUNCTION) Here HEADER
is a header field name (a symbol), and FUNCTION is how to format that
header field, if it appears in HEADERS. Each FUNCTION should take two
arguments: the header symbol, and the value of that header. The value
returned by FUNCTION is inserted in the buffer unless it is nil.

If the function for a header field is nil, or if no function is
specified for a particular header field, the default action is to
insert the value of the header, unless it is nil.

The headers are inserted in the order of the FORMAT-RULES.
A key of t in FORMAT-RULES represents any otherwise unmentioned headers.
A key of nil has as its value a list of defaulted headers to ignore.