Here is page 21 of RFC 2822. Emphasis is mine.

RFC 2822                Internet Message Format               April 2001

   for delivery.  The origination date is intended to contain the date
   and time that the user queued the message, not the time when the user
   connected to the network to send the message.)
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.
Resnick                     Standards Track                    [Page 21]