``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 cake.

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 interface.

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:


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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