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<TITLE>"Reply-To" Munging Considered Harmful</TITLE>
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<h1>This Page is a Mirror</h1>
<p>This page has been mirrored for historical preservation</p>
<hr>
<H1>"Reply-To" Munging Considered Harmful</H1>
<H2>An Earnest Plea to Mailing List Administrators</H2>
<HR>
<P>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
<A HREF="http://www.fwi.uva.nl/~mes/jargon/m/munge.html">munge</A> some
of the message headers.
<P>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 <KBD>Reply-To</KBD> header that
points back to the list. This transformation also is one of the most
ill-advised.
<P>Some administrators claim that <KBD>Reply-To</KBD> munging makes
it easier for users to respond to the entire list, and helps encourage
list traffic. These benefits are fallacious. Moreover, <KBD>Reply-To</KBD>
can have harmful -- even dangerous -- effects. If you think
<KBD>Reply-To</KBD> munging is a good idea, I hope I can change your
mind.
<H2>The Principle of Minimal Munging</H2>
<P>Email processing is pretty tricky. Read through
<A HREF="ftp://ftp.isi.edu/in-notes/rfc822.txt">RFC-822</A>, the
<CITE>Standard for the Format of ARPA Internet Text Messages</CITE>,
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.
<A HREF="ftp://ftp.isi.edu/in-notes/rfc1123.txt">RFC-1123</A>, the
commonly-called <CITE>Internet Host Requirements</CITE> 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.<P>
<P>The "Principle of Minimal Munging" is a good rule that will keep
you out of trouble. It says you should <EM>not</EM> 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.
<P>The "Principle of Minimal Munging" will help you avoid the sorts of
problems we are about to discuss. This principle <EM>is</EM> a rule
designed to be broken, but you can avoid some significant heartache
by thinking hard and long before you do so.
<H2>It Adds Nothing</H2>
<P><KBD>Reply-To</KBD> munging does not benefit the user with a reasonable
mailer. People want to munge <KBD>Reply-To</KBD> 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
<A HREF="http://www.bsdi.com/bsdi-man/?Mail%281%29">Berkeley
<CITE>Mail</CITE></A> command has had this for about a decade.
<P>Any reasonable, modern mailer provides this feature. I prefer the
<A HREF="http://www.myxa.com/myxa/elm.html"><CITE>Elm</CITE> mailer</A>.
It has separate "r)eply" and "g)roup-reply" commands. If I
want to reply to the author of a message, I strike the "<KBD>r</KBD>"
key. If I want to send a reply to the entire list, I hit "<KBD>g</KBD>"
instead. Piece 'o cake.
<P>I mention <CITE>Elm</CITE> here (and a lot later on) simply because
that's the mailer I use everyday. This sort of support is not unique
to <CITE>Elm</CITE> Any reasonable mailer provides it. The
<CITE>Pine</CITE> mailer, for instance, asks directly, "Reply to all
recipients?" when you use the "<KBD>r</KBD>" command. It doesn't
get much easier than that!
<P>Whichever mailer you choose, please <EM>read</EM> 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.
<H2>It Makes Things Break</H2>
<P>If you use a reasonable mailer, <KBD>Reply-To</KBD> munging does not
provide any new functionality. It, in fact, <EM>decreases</EM>
functionality. <KBD>Reply-To</KBD> 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. <KBD>Reply-To</KBD> munging is not merely benign,
it is harmful. It renders a useful mail capability inoperative.
<H2>Freedom of Choice</H2>
<P>Some administrators justify <KBD>Reply-To</KBD> munging by saying,
"All responses should go directly to the list anyway." This is
arrogant. You should allow <EM>me</EM> to decide exactly how I wish
to respond to a message. If I feel a public response is justified,
I'll hit the "<KBD>g</KBD>" key and tell <CITE>Elm</CITE> to do a
group-reply. If I believe a private response is more appropriate,
I'll use "<KBD>r</KBD>" to send one. Please allow me the freedom
to decide how to handle a message.
<H2>Can't Find My Way Back Home</H2>
<P>It may be impossible to reply to the author of a message once the
<KBD>Reply-To</KBD> header is munged. The <KBD>Reply-To</KBD> 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.
<P>There are good reasons why the sender might insert a <KBD>Reply-To</KBD>
header. The sender might <EM>not</EM> be the original author of the
message (the name that appears in the <KBD>From</KBD> header). If
responses should return to the sender and not the original author,
then the sender will insert a <KBD>Reply-To</KBD> header. Or, maybe
the sender added a <KBD>Reply-To</KBD> because he or she cannot receive
email at the account from which the message was sent. There are many
good reasons to place a <KBD>Reply-To</KBD> header into a mailing list
message.
<P>If the <KBD>Reply-To</KBD> is munged by the mailing list, the value
provided by the original sender is lost. <KBD>Reply-To</KBD> munging
can make it impossible to reach the sender of a message.
<H2>Coddling the Brain-Dead, Penalizing the Conscientious</H2>
<P>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 <KBD>Reply-To</KBD>
munging. It makes it easier for him or her to send responses directly
to the list.
<P>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.
<H2>Principle of Least Work</H2>
<P>Compare and contrast: the work required for me (or any other
<CITE>Elm</CITE> user) to reply on lists that do and don't employ
<KBD>Reply-To</KBD> munging.
<BLOCKQUOTE>
<PRE>
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.
</PRE>
</BLOCKQUOTE>
<P>Again, your preferred mailer probably implements this feature in
a different fashion. Nonetheless, it should be easy. I'll take box
number one, Monte.
<H2>Principle of Least Surprise</H2>
<P>When I hit the "<KBD>r</KBD>" key in <CITE>Elm</CITE>, it sends a
response to the author of a message. When you munge the <KBD>Reply-To</KBD>
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 interface.
<P>Private messages frequently are broadcast across lists that do
<KBD>Reply-To</KBD> munging. That's an empirical fact. It's what
happens when you violate the principle of least surprise.
<H2>Principle of Least Damage</H2>
<P>Consider the damage when things go awry. If you do not munge the
<CITE>Reply-To</CITE> 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.
<P>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).
<P><KBD>Reply-To</KBD> munging encourages catastrophic failure modes.
Sure, you don't need <KBD>Reply-To</KBD> 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.
<H2>And in the End...</H2>
<P>If you are not convinced yet, then allow me one final plea. I contribute
to the <CITE>Elm</CITE> 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 <EM>third</EM> reply command
-- one that ignores any existing <KBD>Reply-To</KBD> header! Want to
guess why people are asking for it? If you think you are doing your
subscribers a service by munging <KBD>Reply-To</KBD> headers, you are
kidding yourself. You are making your subscribers miserable.
<P>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?
<H2>Summary</H2>
<P>Many people want to munge <KBD>Reply-To</KBD> 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. <KBD>Reply-To</KBD>
munging suffers from the following problems:
<UL>
<LI>It violates the principle of minimal munging.
<LI>It provides no benefit to the user of a reasonable mailer.
<LI>It limits a subscriber's freedom to choose how he or she will direct
a response.
<LI>It actually reduces functionality for the user of a reasonable mailer.
<LI>It removes important information, which can make it impossible to
get back to the message sender.
<LI>It penalizes the person with a reasonable mailer in order to coddle
those running brain-dead software.
<LI>It violates the principle of least work because complicates the
procedure for replying to messages.
<LI>It violates the principle of least surprise because it changes the
way a mailer works.
<LI>It violates the principle of least damage, and it encourages a failure
mode that can be extremely embarrassing -- or worse.
<LI>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.
</UL>
<H2>Addendum</H2>
<P>In case you are wondering, yes, I once thought <KBD>Reply-To</KBD> munging
was a nifty idea. I got better though.
<P>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.
<P>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.
<P>
<HR>
<ADDRESS>
<A HREF="../people/chip/index.html">Chip Rosenthal</A><BR>
&lt;chip@unicom.com&gt;<BR>
</ADDRESS>
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