I've been hosting and "owning" (nannying, moderating, facilitating...) Green email lists for a dozen years. Every few months someone asks/suggests/demands that I set the list(s) she is on to alter every message that goes through it. The alteration she wants is that the mailing list software should add a line to the message headers. That line should say "Reply-To: posting address".

The reason she wants the mailing list software to do that is that she is having trouble posting to the list. When she is looking at a message that came through the list, in her colorful and oh-so-user friendly email program, and she hits its big colorful "Reply" button, her email program opens a composing form. (Hey! The term for the computer program you use to read and send email is mail user agent or MUA. Microsoft Outlook Express is an MUA. So is the part of Yahoo Mail that you interact with.) The composing form has a To: line with the author of the message filled in. She wanted to post a followup to the list, not to the previous poster in private. She is having trouble changing that To: line so her message will go to the list.

That's not a problem with the mailing list configuration. It's a problem with her user friendly MUA.

There are problems with her suggested fix.

1. It won't work. Some MUAs don't respect the Reply-To: line. Some add the reply-to address, but they leave the from address in there as well. Some just ignore it. Some perverse MUAs might ignore it, but propagate it into subsequent replies.
   
2. It messes up some message transport software. When you email a friend, the message you send only has to work with your MUA, her MUA, and the particular software your service provider and hers use. But messages from a mailing list have to work with every message transport and every MUA that any subscriber might be suffering with. What happens when a "reply-to list" modified message hits a broken message transport serving an abandoned address? Believe it or not, there are broken message transports in wide use that will return the non-delivery notice ("bounce") to the Reply-To: address. What happens when that bounce notice gets reposted, and has to bounce again? Most mailing list programs are smart enough to detect the "looping" and prevent all the subscribers from getting mailbombed. But we'd rather not take the chance.
   
3. It makes the list noisier. Decades of experience with many thousands of email lists shows that lists that work the right way (default reply to original author) are more focused. If it takes just a tad more effort to post to the list than to reply to the last poster, then side remarks, angry knee-jerk responses, and off topic digressions are harder to do. This works.
4. It creates an opportunity for confusion and mischief. What happens when you post the same message to two mailing lists? (You shouldn't, but mailing lists have to handle the problem of people who insist on it.) Where should the replies go? Should a subscriber to only one list be left unaware that she's not replying to both?
   

The right way to fix the problem is for the subscribers to take personal responsibility. Hey! That's one of the Ten Key Values of the (USA) Greens. To use email responsibly, you need to be able to control where you send each message. In some MUAs, it is rather difficult to copy an email address out of a message header and paste it into your address book. But it's not that hard. Make an address book entry for that posting address. Sometimes you might need to save your new message as a draft, and then send slightly different versions of it different places. That's a good habit to learn. It will save you time and confusion.

And now that you've stopped procrastinating and decided to spend a few minutes in your MUA's built-in help, or googling for information that's missing from there, here are two more things worth learning.

1. Every MUA I've tried in the last fifteen years has a way to show the message headers. Find yours. Most mailing lists show their posting address in a message header. You can select it from there instead of fumbling around in the mailing list's web pages. They usually show the unsubscribe address, too. When you get a message that says it's from your bank or Ebay, you can open the headers and find out for sure. But you'll never take that precaution if you didn't already find the show headers button or menu item. This knowledge could keep you from getting phished.
   
2. Stop sending a Reply-To: header line, at least by default. Not only do they screw up broken message transports and mailing list bounce collection, but they interfere with some of your recipients' MUAs. Maybe your recipient wants to hit Reply-to-all in her user friendly MUA. Sometimes that's appropriate. Your Reply-To may override her wishes. Give her some credit for knowing what she's doing. If your MUA is adding a Reply-To line to the messages you send, and you can't turn it off, your MUA is broken. If you turned it off in the default settings, and you can't easily add one when you want to, your MUA is broken. If your MUA is broken, you should report a bug to its author. If your MUA has no way to report bugs, it's really broken and can only get worse, and you should switch to a well maintained MUA as soon as you can.
   

P.s., The English language doesn't provide a gender-neutral, singular, third person pronoun for a person. It is no good. I'm using she and her in this article. Historically, we've used he and him. Call it affirmative action for oppressed pronouns. But don't take it to mean I'm talking about women. Men request the reply-to-list munge just about as often as women do.

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