2.1.4 Header field: Reply-To
Description:
Mailbox for replies to message
That's not quite correct. Reply-To specifies a *list* of *addresses* for
replies,
as opposed to a *single* *mailbox*. Addresses include named groups as well as
mailboxes.
According to my notes, here is what the various address fields specify:
RFC 2822:
from = "From:" mailbox-list CRLF
sender = "Sender:" mailbox CRLF
reply-to = "Reply-To:" address-list CRLF
to = "To:" address-list CRLF
cc = "Cc:" address-list CRLF
bcc = "Bcc:" (address-list / [CFWS]) CRLF
RFCs 724, 733 permitted address-lists in From
Note that Reply-To is consistent with To, Cc, and Bcc. All of which
permit anything that might appear in From (but not vice-versa since
RFC 822 obsoleted RFC 733). Sender permits only a single mailbox.