rfc2822 April 2001

3.6.2. Originator fields

   The originator fields of a message consist of the from field, the
   sender field (when applicable), and optionally the reply-to field.
   The from field consists of the field name "From" and a
   comma-separated list of one or more mailbox specifications.  If the
   from field contains more than one mailbox specification in the
   mailbox-list, then the sender field, containing the field name
   "Sender" and a single mailbox specification, MUST appear in the
   message.  In either case, an optional reply-to field MAY also be
   included, which contains the field name "Reply-To" and a
   comma-separated list of one or more addresses.

from            =       "From:" mailbox-list CRLF

sender          =       "Sender:" mailbox CRLF

reply-to        =       "Reply-To:" address-list CRLF

   The originator fields indicate the mailbox(es) of the source of the
   message.  The "From:" field specifies the author(s) of the message,
   that is, the mailbox(es) of the person(s) or system(s) responsible
   for the writing of the message.  The "Sender:" field specifies the
   mailbox of the agent responsible for the actual transmission of the
   message.  For example, if a secretary were to send a message for
   another person, the mailbox of the secretary would appear in the
   "Sender:" field and the mailbox of the actual author would appear in
   the "From:" field.  If the originator of the message can be indicated
   by a single mailbox and the author and transmitter are identical, the
   "Sender:" field SHOULD NOT be used.  Otherwise, both fields SHOULD
   appear.

   The originator fields also provide the information required when
   replying to a message.  When the "Reply-To:" field is present, it
   indicates the mailbox(es) to which the author of the message suggests
   that replies be sent.  In the absence of the "Reply-To:" field,
   replies SHOULD by default be sent to the mailbox(es) specified in the
   "From:" field unless otherwise specified by the person composing the
   reply.

   In all cases, the "From:" field SHOULD NOT contain any mailbox that
   does not belong to the author(s) of the message.  See also section
   3.6.3 for more information on forming the destination addresses for a
   reply.