A header field in the mail buffer starts with a field name at the beginning of a line, terminated by a colon. Upper and lower case are equivalent in field names. After the colon and optional whitespace comes the contents of the field.
You can use any name you like for a header field, but normally people use only standard field names with accepted meanings.
The ‘From’ header field identifies the person sending the email
(i.e., you). This should be a valid mailing address, as replies are
normally sent there. The default contents of this header field are
computed from the variables
user-full-name (which specifies
your full name) and
user-mail-address (your email address). On
some operating systems, Emacs initializes these two variables using
environment variables (see General Variables). If this
information is unavailable or wrong, you should customize the
variables yourself (see Easy Customization).
Apart from ‘From’, here is a table of commonly-used fields:
The mailing address(es) to which the message is addressed. To list more than one address, use commas to separate them.
The subject of the message.
Additional mailing address(es) to send the message to. This is like ‘To’, except that these readers should not regard the message as directed at them.
Additional mailing address(es) to send the message to, which should not appear in the header of the message actually sent. ‘BCC’ stands for blind carbon copies.
The name of a file, to which a copy of the sent message should be appended. Emacs writes the message in mbox format, unless the file is in Babyl format (used by Rmail before Emacs 23), in which case Emacs writes in Babyl format. If an Rmail buffer is visiting the file, Emacs updates it accordingly. To specify more than one file, use several ‘FCC’ fields, with one file name in each field.
An address to which replies should be sent, instead of ‘From’. This is used if, for some reason, your ‘From’ address cannot receive replies.
This field takes precedence over ‘Reply-To’. It is used because some mailing lists set the ‘Reply-To’ field for their own purposes (a somewhat controversial practice).
One of more address(es) to use as default recipient(s) for follow-up messages. This is typically used when you reply to a message from a mailing list that you are subscribed to, and want replies to go to the list without sending an extra copy to you.
An identifier for the message you are replying to. Most mail readers use this information to group related messages together. Normally, this header is filled in automatically when you reply to a message in any mail program built into Emacs.
Identifiers for previous related messages. Like ‘In-Reply-To’, this is normally filled in automatically for you.
The ‘To’, ‘CC’, and ‘BCC’ fields can appear any number of times, and each such header field can contain multiple addresses, separated by commas. This way, you can specify any number of places to send the message. These fields can also have continuation lines: one or more lines starting with whitespace, following the starting line of the field, are considered part of the field. Here’s an example of a ‘To’ field with a continuation line:
To: email@example.com, firstname.lastname@example.org, email@example.com
You can direct Emacs to insert certain default headers into the mail
buffer by setting the variable
mail-default-headers to a
string. Then C-x m inserts this string into the message
headers. For example, here is how to add a ‘Reply-To’ and
‘FCC’ header to each message:
(setq mail-default-headers "Reply-To: firstname.lastname@example.org\nFCC: ~/Mail/sent")
If the default header fields are not appropriate for a particular message, edit them as necessary before sending the message.