Reply-to-List, or Reply-to-All?

There is an open question about the best "netiquette" to use when replying to mailing list messages — should you use "Reply to All" and include the original sender of the message and all other recipients well as the list, or should you use "Reply to List" and send your message only to the list?

Note that this is not the same topic as discussed in Chip Rosenthal's "Reply-To" Munging Considerered Harmful" essay. That essay is about what the list should do, while this page talks about your choices as the user who is replying to the mail.

Background

Firstly, it's important to understand how mailing list software behaves. One of the most commonly used mailing list managers is GNU Mailman.

By default, Mailman will not send you a copy of a list email if it sees that your address is already listed in the To: or Cc: headers of the email. This can be changed by logging into the list web site and changing the duplicates setting.

Different people have different preferences, use different software, and have different ways of processing their incoming mail. For example:

• John doesn't do any filtering — he isn't subscribed to many mailing lists, so hasn't bothered to work out how to do it. The default behaviour of Mailman means that if you Cc him on a message which is also sent to a list he's on, he'll just get one copy of it in his INBOX.
• Susan also doesn't do any filtering; she is subscribed to the daily list digest to keep the amount of mail traffic down.
• Pete filters mailing list messages into separate folders, and and has also kept the default "no duplicates" list configuration. His filters match the List-Id: header of an incoming message to determine whether it came from the list. So if you send a message to both Pete and the list, he will receive that message in his INBOX rather than the list folder.
• James's set up is like Pete's, but his filters are slightly different. His filters just trigger on the list address being in the To: or Cc: headers. So if you Cc him on a message to the list, it will land in his list folder not his INBOX.
• Fred filters all incoming messages into separate folders according to the list it comes from, and has told the list software that he does want to receive the so-called "duplicates". If you send a message which is cross-posted to two lists that he's subscribed to, and Cc him directly because you know he has a personal interest and you don't know that he's subscribed (or suspect he might not be looking in those folders very often), then he will have separate copies in each list folder, in addition to the copy in his INBOX that he sees immediately.
• Claire is not subscribed to the list at all. She's been added to Cc because she has an interest in this particular discussion, and can offer expert advice even though she's not usually involved with this project at all.
• Karl is not subscribed to the same list as you; he's subscribed to a different mailing list. If you are participating in a discussion which crosses project boundaries and is cross-posted to more than one list, he will only see messages which are posted to the list he's subscribed to.

Obviously if they have told you their preference, then that's not so hard — just do as they ask. But most of the time you don't know; you have to guess. And when we're working on mail software, we have to set the default behaviour for a non-private reply. That's what prompted me to write this page.

Pros and Cons

• Claire is cut out of the discussion entirely.
• Susan doesn't get to see the message until the next digest is sent. Which is usually OK as that's what she signed up for, but if she has actively participated in the thread then she's now not going to see your response until much later when the discussion has already moved on.
• Karl, and the entire mailing list that he's on, are also cut out of the discussion.
• Fred is unhappy because although you're replying to a thread he's actively involved in, and you may even be responding directly to something he's said, you didn't do him the courtesy of a direct Cc so it doesn't land in his INBOX. So he doesn't see it immediately, and like Susan he might not actually spot it in the list folder until much later when the discussion has moved on.
• John is happy. The message lands in his INBOX.
• James is happy. His filters put the message in his list folder.
• Pete is happy, because his filters will put the message into his list folder where he wants it.

• Claire is happy. She was explicitly added to Cc because her input is valued, and she continues to be part of the discussion.
• Susan is happy. Where she has actively participated in a thread, she is included in that discussion in real time.
• Karl is happy. He may or may not be following the the list that he's on at this precise moment in time, but the full discussion is there in his list folder and doesn't disappear off into different forks in different places.
• Fred is happy. The messages are in the correct list folder(s) for posterity, and there's a copy in his INBOX which he'll see and can respond to immediately.
• John and James are happy. The result is the same for them either way.
• Pete is unhappy, because he wants the message to go into the list folder but it lands in his INBOX. He's also unhappy because it doesn't have the List-Post: header which lets him choose the "Reply to List" action on it.

Your decision

Now it's time for you to make your own decision — if you don't know what the sender prefers, which one do you guess?

The corollary to the same question is: If you're working on a mail program, what should you make the default behaviour, if the user doesn't express a preference?

I've tried to stick to the facts so far and not offer my opinion. If you don't want to know what I think, you can look away now...

• If you "Reply to All" then some people may have configured their filters to put the message in the wrong place. Or if it's a list which doesn't automatically avoid the "duplicates" like Mailman does, they might get a second copy of the message. Neither of these are really the end of the world.
• If you "Reply to List" then some people could be cut out of the discussion completely, and for a cross-posted discussion you risk splitting the thread into separate discussions on the different lists.

Duplicates

Note that even if people are sent multiple copies of a message via a substandard list manager which doesn't allow them to filter duplicates, often they won't see them anyway because many mail stores automatically eliminate messages with the same Message-Id header, since that's supposed to be unique. (This isn't a stunningly good idea, and Fred gets really unhappy if forced to use such a system, but it is distressingly common).

Missing List-* headers

Pete was also unhappy that he couldn't use "Reply to List" on the message which he received directly, because it doesn't have the list headers. But we've just established that that's a benefit for everybody else since he shouldn't be doing that anyway!

However, it does also mean that for such messages you don't get to use the automated features like being able to unsubscribe using the details in the List-Unsubscribe: header, and other such things. Did you even know that was possible? When compared with the possibility of cutting people out of the discussion, I think we can agree that this is probably worth it. It's not as if it's hard to just click on a different thread in the list to get those headers. So you're actively participating in every thread in the list and it's hard to find a message with the list headers? Tell me... why did you want to unsubscribe, again? :)

Filter setup

Pete might not like being told that his filters are wrong. He might say that filtering on To: and Cc: headers like James does has false positives, and files messages into a list folder when they didn't really come via the list. Pete is theoretically correct to a certain degree, but filtering on the List-Id: header also has false positives in some circumstances. The best way to filter list traffic is actually on the Return-Path.

When this is pointed out, Pete will usually shrug his shoulders and say that those circumstances don't happen very often and using the List-Id: works well enough for him. That is probably the point at which we might suggest to him that, while James's method also has theoretical false positives, those are actually making it do the right thing for him most of the time — so Pete is probably better off using it.

I said "most of the time"... Pete might have a valid concern that if a message is sent to more than one mailing list that he's on, his filters might put it in the wrong place if he only filters on the contents of the To: and Cc: headers.

We should stop here and think for a second. What happens if we pander to Pete's demands that we all use "Reply to List"? That's right — we end up continuing the thread only on one of the mailing lists, fairly much randomly chosen from Pete's point of view, so he still gets the messages only in one of the folders! It's just that everyone else is inconvenienced, some in much greater ways, too!

But let's tell Pete how to fix it anyway. The best option, if you really want this setup for your email, is a combination of filters in two passes. First, do the "proper" filtering on the Return-Path of each list, so that any message which actually does arrive via a list is reliably filed into the appropriate folder for that list. And then, if you really don't want list conversations landing in your INBOX even after you've actively participated in them, your filters can also match on the To: and Cc: headers and filter matching mails into the appropriate list folders.

But maybe it's not worth bothering with that second pass. It's not like it's hard to move messages from your INBOX to a list folder if you want that to happen. And you might get used to the fact that when people are replying to your messages in a thread, you get to see their response and respond to it immediately rather than later on.

Conclusion

So there it is. Unless you know that the other people involved in the discussion are like Pete (and either can't or won't fix their own filters) — or that the list doesn't eliminate "duplicates" and they object to them — then you should always play it safe and use "Reply to All".

Deliberately replying only to one list and cutting others out of the discussion is considered to be extremely rude by some people. When they eventually find out.

Public vs. Private replies

• If you send a private reply when the user expected it to be public, that is easy to fix — just forward or resend the message to the correct place.
• If you send a public reply when the user expected it to be private, the effects of that can be catastrophic — the public message can never be retracted, and could be extremely damaging.

Any decent mail program will have separate options for a private reply, vs. a more public reply — and the private reply option should never trick you into replying back to the list.

Reply-To: list

Please, never set a mailing list to have a Reply-To: header point back to the list. That's like sending me a virus which infects my mail program and hacks it so that it replies in public when I specifically pressed the button which should give me a private reply. If I wanted to send a public reply, the button for that is right next to the one I pressed! If I'd wanted that, I would have pressed it. Thankfully, I now have this to save me:

Evolution

You don't have to use the drop-down menu, of course — if you just click 'Group Reply', it'll do the sane default which is Reply-to-All. You can go into the preferences and change the default to be Reply-to-List. Please don't, though.