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Reply-to munging considered harmful

From: Karl Fogel <kfogel_at_galois.collab.net>
Date: 2000-09-12 16:27:21 CEST

Jason, et al,

Can you tell me how to make dev@subversion.tigris.org not munge
Reply-to headers forcing reply to the list? I'd like to make it
convenient to reply to either author alone or the entire list (using
the "follow-up" feature of any mailer), which is what we'll get if we
simply don't touch the Reply-to header.

I was going to write a long essay explaining why I think this is
important, but then Jim Blandy (in a mail to me requesting exactly
this change) pointed out this URL


which says exactly what I wanted to say (I've attached the content

I actually think this would be a good change for all Tigris lists, but
if not, let's please at least do it for Subversion (just point me to
the right place and I'll make the change). For what it's worth,
*several* people have emailed me requesting this, not just Jim, and I
also frequently (really, on 90% of my replies to dev posts) find
myself manually cutting-and-pasting to achieve reply-to-author-alone.

Here's the text at that URL:

``Reply-To'' Munging Considered Harmful

An Earnest Plea to Mailing List Administrators

An email message requires some amount of processing when it is
redistributed to a mailing list. At the very least, the envelope must
be rewritten to redirect bounces directly to the list
administrator. While the message is being processed, the list
administrator might take advantage of the opportunity to munge some of
the message headers.

Some forms of header munging are helpful, such as special
loop-detection headers. Others are questionable. Most are ill-advised
or dangerous. Many list adminstrators want to add a Reply-To header
that points back to the list. This transformation also is one of the
most ill-advised.

Some administrators claim that Reply-To munging makes it easier for
users to respond to the entire list, and helps encourage list traffic.
These benefits are fallacious. Moreover, Reply-To can have harmful --
even dangerous -- effects. If you think Reply-To munging is a good
idea, I hope I can change your mind.

The Principle of Minimal Munging

Email processing is pretty tricky. Read through RFC-822, the Standard
for the Format of ARPA Internet Text Messages, sometime. It is 47
pages of dense, dry detail. A lot of engineering and consideration
went into this work. Even still, RFC-822 leaves many corner conditions
and specialized circumstances poorly specified. RFC-1123, the
commonly-called Internet Host Requirements document, adds a couple
dozen more pages, and remedies some of the defects. Then there is
MIME, X.400 mapping, and a handful of other standards and conventions
-- some documented and some folklore. Email handling is surprisingly
complicated, and even an innocuous-sounding change might have grave,
unintended consequences.

The ``Principle of Minimal Munging'' is a good rule that will keep you
out of trouble. It says you should not make any changes to an email
header unless you know precisely what you want to do, why you want to
do it, and what it will affect. Unless you can articulate a clear
reason for munging and understand the full consequences of the action,
you should not do it.

The ``Principle of Minimal Munging'' will help you avoid the sorts of
problems we are about to discuss. This principle is a rule designed to
be broken, but you can avoid some significant heartache by thinking
hard and long before you do so.

It Adds Nothing

Reply-To munging does not benefit the user with a reasonable
mailer. People want to munge Reply-To headers to make ``reply back to
the list'' easy. But it already is easy. Reasonable mail programs have
two separate ``reply'' commands: one that replies directly to the
author of a message, and another that replies to the author plus all
of the list recipients. Even the lowly Berkeley Mail command has had
this for about a decade.

Any reasonable, modern mailer provides this feature. I prefer the Elm
mailer. It has separate ``r)eply'' and ``g)roup-reply'' commands. If I
want to reply to the author of a message, I strike the ``r'' key. If I
want to send a reply to the entire list, I hit ``g'' instead. Piece 'o

I mention Elm here (and a lot later on) simply because that's the
mailer I use everyday. This sort of support is not unique to Elm Any
reasonable mailer provides it. The Pine mailer, for instance, asks
directly, ``Reply to all recipients?'' when you use the ``r''
command. It doesn't get much easier than that!

Whichever mailer you choose, please read the fine manual that comes
with it. Unless you are stuck with some decrepit mail system, I bet
you'll find it has a similar feature. If so, you easily can choose to
direct your responses either to the original author or the entire
list. Mauling the mail headers doesn't make it any easier.

It Makes Things Break

If you use a reasonable mailer, Reply-To munging does not provide any
new functionality. It, in fact, decreases functionality. Reply-To
munging destroys the ``reply-to-author'' capability. Munging makes
this command act effectively the same as the ``reply-to-group''
function. We haven't added anything new, we've only taken
away. Reply-To munging is not merely benign, it is harmful. It renders
a useful mail capability inoperative.

Freedom of Choice

Some administrators justify Reply-To munging by saying, ``All
responses should go directly to the list anyway.'' This is
arrogant. You should allow me to decide exactly how I wish to respond
to a message. If I feel a public response is justified, I'll hit the
``g'' key and tell Elm to do a group-reply. If I believe a private
response is more appropriate, I'll use ``r'' to send one. Please allow
me the freedom to decide how to handle a message.

Can't Find My Way Back Home

It may be impossible to reply to the author of a message once the
Reply-To header is munged. The Reply-To header was not invented on a
whim. It is there for the sender of a mail message to use. If you
stomp on this header, you can lose important information.

There are good reasons why the sender might insert a Reply-To
header. The sender might not be the original author of the message
(the name that appears in the From header). If responses should return
to the sender and not the original author, then the sender will insert
a Reply-To header. Or, maybe the sender added a Reply-To because he or
she cannot receive email at the account from which the message was
sent. There are many good reasons to place a Reply-To header into a
mailing list message.

If the Reply-To is munged by the mailing list, the value provided by
the original sender is lost. Reply-To munging can make it impossible
to reach the sender of a message.

Coddling the Brain-Dead, Penalizing the Conscientious

There are, unfortunately, poorly implemented mail programs that lack
separate reply-to-author and reply-to-group functions. A user saddled
with such a brain-dead mailer can benefit from Reply-To munging. It
makes it easier for him or her to send responses directly to the list.

This change, however, penalizes the conscientious person that uses a
reasonable mailer. This is a poor trade-off. As Internet list
administrators, we should encourage people to run reasonable
software. If a few people need to type in a full reply address so that
everybody else can use all the features of their mailer, I say,
``Fine!'' We should not penalize the conscientious to coddle those who
run brain-dead software.

Principle of Least Work

Compare and contrast: the work required for me (or any other Elm user)
to reply on lists that do and don't employ Reply-To munging.

                     Case One: Case Two:
     Action Without Munging With Munging
     ============= ===================== =====================

     Reply to Hit the "g" Probably hit the "r"
     everybody. key. key, but maybe the "g"
                                             key if there were other
                                             recipients of the message.

     Reply just Hit the "r" Look at the original
     to author. key. message header, write
                                             down the sender's
                                             email address, hit the
                                             "r" key, call up the
                                             header editing menu,
                                             erase the current To:
                                             value, and type in the
                                             sender's full email
                                             address. And pray the
                                             correct address wasn't
                                             wiped out when the Reply-To
                                             was munged.

Again, your preferred mailer probably implements this feature in a
different fashion. Nonetheless, it should be easy. I'll take box
number one, Monte.

Principle of Least Surprise

When I hit the ``r'' key in Elm, it sends a response to the author of
a message. When you munge the Reply-To header you change this action
so that it does something entirely different from what I expect. This
creates specialized behavior for your mailing list, which increases
the potential for surprise. I'm not schooled in the science of human
factors, but I suspect surprise is not an element of a robust user

Private messages frequently are broadcast across lists that do
Reply-To munging. That's an empirical fact. It's what happens when you
violate the principle of least surprise.

Principle of Least Damage

Consider the damage when things go awry. If you do not munge the
Reply-To header and a list subscriber accidentally sends a response
via private email instead of to the list, he or she has to follow up
with a message that says, ``Ooops! I meant to send that to the
list. Could you please forward a copy for me.'' That's a hassle, and
it happens from time to time.

What happens, however, when a person mistakenly broadcasts a private
message to the entire list? If the message is a complaint about the
personal hygiene of sender's boss, or the sex life of his or her
roommate, a simple ``Ooops!'' won't cut it. About all you can do is
send a followup with lots of retroactive smileys (weak). Or say your
cat was dancing on the keyboard (better). Or start reading the
classifieds for a new job/roommate/set of teeth (most likely).

Reply-To munging encourages catastrophic failure modes. Sure, you
don't need Reply-To munging to create this sort of damage. A simple
slip of the fingers will suffice. When, however, you violate the
``Principle of Least Surprise'' you invite this sort of disaster. A
responsible list administrator will avoid creating avenues that lead
to such extreme damage.

And in the End...

If you are not convinced yet, then allow me one final plea. I
contribute to the Elm mailer development team. I get to see a lot of
the wants and requests from the user community. Guess what feature
more and more people are asking for? A third reply command -- one that
ignores any existing Reply-To header! Want to guess why people are
asking for it? If you think you are doing your subscribers a service
by munging Reply-To headers, you are kidding yourself. You are making
your subscribers miserable.

Some list administrators, even after reading all this, seem to say,
``Oh, it's not that bad. Besides, my subscribers like it!'' If they
do, it's probably because they haven't bothered to learn to use the
``reply-to-group'' feature of their mailer. Instead of going through
all the trouble of making your list gateway scribble on email headers,
how about making an effort to educate your subscribers?


Many people want to munge Reply-To headers. They believe it makes
reply-to-list easier, and it encourages more list traffic. It really
does neither, and is a very poor idea. Reply-To munging suffers from
the following problems:

     It violates the principle of minimal munging.
     It provides no benefit to the user of a reasonable mailer.
     It limits a subscriber's freedom to choose how he or she will
       direct a response.
     It actually reduces functionality for the user of a reasonable mailer.
     It removes important information, which can make it impossible to
       get back to the message sender.
     It penalizes the person with a reasonable mailer in order to
       coddle those running brain-dead software.
     It violates the principle of least work because complicates the
       procedure for replying to messages.
     It violates the principle of least surprise because it changes
       the way a mailer works.
     It violates the principle of least damage, and it encourages a
       failure mode that can be extremely embarrassing -- or worse.
     Your subscribers don't want you to do it. Or, at least the ones
       who have bothered to read the docs for their mailer don't want
       you to do it.


In case you are wondering, yes, I once thought Reply-To munging was a
nifty idea. I got better though.

When I started running email lists, I munged 'em all. One day I
accidentally sent a private, personal reply out over one of my own
damn lists. If the list owner can't remember how to use the list
properly, no way will the subscribers be able to sort it out. I
stopped munging the very next day.

On the whole, it has worked out quite well. Yes, on occasion somebody
mistakenly responds directly to the author of a message when they
wanted to reply to the group. Most folks, however, seem to catch on
pretty fast to how it works, and seem to appreciate the flexibility.
Moreover, private responses mistakenly sent to the entire list have
become an almost unheard-of event.

What do You Think?

Leave your comments and see what others say in the discussion forum.

Chip Rosenthal

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$Id: reply-to-harmful.html,v 1.17 1999/01/05 09:08:03 chip Exp $
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# Revision 1.17 1999/01/05 09:08:03 chip
# fixed links to RFCs
# started logging modification history
Received on Sat Oct 21 14:36:08 2006

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