.sh 2 "Message headers"
.pp
In section 2, we saw that the
.b reply
command uses the
.q "To:"
and
.q "Cc:"
fields of the original message to construct the recipient list.
In section 3.1, we saw the ~s, ~t, and ~c escapes for modifying
the
.q "Subject:"
.q "To:"
and
.q "Cc:"
fields.  In addition to these headers,
.i Mail
allows, but does not use, other header fields which may be defined
by, say, the ARPANET message standard.  Any header field, whether
supported by
.i Mail
or not, can be modified using the ~e or ~v escape.  The headers
are present in the message written to the editor and are recovered
after the editor finishes.
.pp
If you desire to send messages using a fuller complement of the
ARPANET standard headers, you can set the binary
.q "askarpa"
option:
.(l
set askarpa
.)L
This will cause
.i Mail
to supply correct header fields for the
.q "Date:"
.q "Sender:"
and
.q "Message-ID:"
fields and to prompt you for the
.q "Subject:"
.q "Reply-To:"
.q "To:"
.q "Cc:"
.q "Comment:"
and
.q "In-Reply-To:"
fields.  For example, we might construct the following headers using
.q askarpa"
.(l
Date: 18 Nov 1979 2320-PDT
From: Kurt at Berkeley
Subject: An example set of headers
Sender: Otherperson at Berkeley
Reply-To: vax.kurt at Berkeley
To: dave george at bbna
Cc: Al Newman at mad-host
Comment: This is much too complex
In-Reply-To: Your questions about headers
Message-ID: 32760.kurt.CSVAX.2320
.)L
The format and meaning of these fields is dictated by the ARPANET
message standard, which is given in the ARPANET Protocol Handbook.
.pp
When
.i Mail
prompts for a field, you can type in a longer than line width field
by ending each line to be continued with a \e.
.i Mail
indicates continuations by indenting the following lines 4 spaces.
For example:
.(l
Subject: The solution to a difficult and important \e
    problem has finally presented itself
.)L
The \e does not actually appear in the real message \*-
.i Mail
will split the header fields so that they will fit on 72 character lines.
