Multiple In-Reply-To

Question

Mail user agents usually display threads of Emails by chaining messages together according to the In-Reply-To and References header fields that contain the Message-IDs of other messages. Although a mail usually only replies to one other message, it may be the case that one message answers multiple others. RFC 5322 allows multiple entries in both fields. What can I expect when I send an email that References or is In-Reply-To multiple IDs this way?

• Is it good practice to do so?
• Does it confuse widespread MUAs?
• Is there any common ground on how to display such a message in a threaded view?

Camise

Answer 1

The "In-Reply-To:" field will contain the contents of the "Message-ID:" field of the message to which this one is a reply (the "parent message"). If there is more than one parent message, then the "In-Reply-To:" field will contain the contents of all of the parents' "Message-ID:" fields. If there is no "Message-ID:" field in any of the parent messages, then the new message will have no "In- Reply-To:" field.

Technically there COULD be a reason where you would reply to multiple emails and it would be valid to place multiple message ids in the In-Reply-To header. I can’t think of any program that actually supports this. As to MUAs they won’t care the delivery that the MUA cares about is the To, Cc, Bcc headers.

The In-Reply-To header and References header would control how threads are displayed. Not sure if any mail clients would have an issue handling the multiple In-Reply-To headers. 99% of the time there would only be a single message ID in the In-Reply-To header. So it’s feasible mail applications won’t support it. However they would support the additional reference entries. And this shouldn’t pose an issue.

Ignatius

Answer 2

What can I expect when I send an email that References or is In-Reply-To multiple IDs this way?

Is it good practice to do so?

As you said the current standard (RFC 5322) defines that, so why should it be a problem to adhere to that standard? Also note that multiple IDs in References is typical, while it's not very common for In-Reply-To (see section "3.6.4. Identification Fields" of RFC 5322).

Does it confuse widespread MUAs?

Like above: A MUA that does not support the standard is not worth using it IMHO. However most MUAs make it very hard to impossible to reply to multiple messages at once; one exception I know is using Emacs' Supercite.

Also make sure you don't mix things: The In-reply-to header field does not specify to which senders the message is being sent; instead it merely adds a reference. Details can be found in section "3.6.4. Identification Fields" of RFC 5322.

In addition I think one can confuse users much more than MUAs when not using the MUA's "reply" function when replying to a message (and using "compose" instead.

Is there any common ground on how to display such a message in a threaded view?

Obviously the graph of messages is a DAG (Directed A-cyclic Graph), with the special case of tree in most cases. So the question is mostly equivalent to "How to display a DAG or tree?" In text-based interfaces you could do it by indenting the child messages relative to their parent messages, possibly allowing to collapse and/or expand child nodes. The latter is probably much more comfortable and easier to grasp in graphical user interfaces.

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