bug#22014: 24.4; RFC 5322 Disallows Multiple 'To' Fields in Mail Headers

[Barak A. Pearlmutter]

According to the info page on mail headers,
http://www.gnu.org/software/emacs/manual/html_node/emacs/Mail-Headers.html

  The ‘To’, ‘CC’, and ‘BCC’ fields can appear any number of times ...

This is in contravention to RFC 5322, the current standard for mail
headers.  See the "Max Number" column of the "to" row in the table on
page 20 (sec 3.6), https://tools.ietf.org/html/rfc5322 which reads "1",
and not just for "to" but also in the "cc" and "bcc" rows.

Some smtp servers---the one @pearlmutter.net's mx in particular as it
happens---check for this and reject messages with multiple 'To' fields
in the header.  I personally would urge the authors of such servers to
reconsider this decision.  On the other hand, it would be nice if Emacs
tried to be conformant in this regard, by detecting and consolidating
multiple To:, CC:, or BCC: fields.  Emacs already scans these while
constructing the recipients list, so the extra logic would be minimal.
Or just (yes-or-no-p "Multiple To, CC or BCC fields, abort ") to let the
user know that something on-standards-conformant is in the air.

                                        --Barak.
--
Barak A. Pearlmutter
 Dept Comp Sci, Maynooth University, Co. Kildare, Ireland
 http://barak.pearlmutter.net