Email Etiquette: Replying to Mailing Lists

Most email clients support the idea of “threaded messages”… that is, the client can group together individual email messages that are part of the same conversation, or “thread”. This is a particularly useful feature on mailing lists where multiple conversations are happening at the same time. For example, the following screenshot of the OpenID General Discussion list shows two distinct message threads in Apple’s Mail application:

Screenshot of Mail.app showing threaded messages

In order to match up messages that belong to the same conversation, email clients use two techniques. First, they match emails with the same subject line, which is mostly accurate. Occasionally this will result in “false positives” where unrelated emails are grouped together because the senders happened to use the same subject. The second method is far more accurate and makes use of a hidden portion of the email message which most people never see. Emails, just like web page requests include a number of hidden “headers” that carry extra information about the message.

Screenshot of email message showing detailed SMTP headers

This includes information about the email client the sender used, the message priority, and the different servers that handled the message along the way. Each email message also includes a unique identification string called the “Message-Id”. This string is unique for every email message in the world… no two are ever identical. In the screenshot above, the Message-Id is <49518270.7010405@degeneration.co.uk>. If someone were to reply to that message, the Message-Id of the original email would be included in the reply email as another hidden header aptly named In-Reply-To:

Screenshot of email reply showing 'In-Reply-To' header

Any future replies would continue to include an In-Reply-To header with this value so that email clients can recognize that they all belong to together and group them accordingly. This method of conversation tracking is far more accurate, but has one problem. That problem occurs when some lazy person comes along and wants to send a message to the mailing list, but instead of clicking on “New Message” in their mail client, they pick a message at random from the list, click “Reply”, and then clear out the subject and body of the message. While this looks like a new message about a new topic, because the person “replied” to a previous email instead of starting a new message, the previous Message-Id got attached. Note the following reply to the same email above, but now the subject has been changed from talking about a Google sign-in button, to the BBC being an OpenID provider. These are completely different discussions!

Screenshot of email message posted as a reply to a previous message, but with a
  new Subject and a new line of discussion

If we go back to our first screenshot of the two message threads, we can see the this new message (as well as any of its replies) get lumped right together with the original discussion.

Screenshot of email thread, highlighting where a new discussion starts in the
  same thread

So today’s lesson in email etiquette: If you’re not actually replying to a particular email, DON’T CLICK “REPLY”. Copy and paste the “To:” email address if you have to, but don’t click reply! There might be some grace here for emails between individuals, but not on mailing lists, especially not on technical mailing lists. (Not meaning to pick on Peter in the screenshots above… he is not the only offender on the OpenID mailing lists, that just happened to be the most recent occurrence).

Okay, that’s my rant for the day… I’m good now. Oh, and Merry Christmas. :)