Trent Mick wrote to me some time ago asking for a feature on this blog: could I make it so that email notifications of blog comments would thread together nicely?
The email subject lines from my notifications look like this:
A comment on "Weird URL data encoding" from Richard Schwartz
I use Thunderbird for email, and don't thread my inbox, so I never considered threading. Trent sent along information from a friend which said that "References:" headers were the key that would make a set of emails into a single thread.
I hacked for a little while, and could not get them to thread. I created a fake message id from the blog post and had all comment notifications have a References header with the id in it. No threading. I added unique Message-ID headers to each comment, then made subsequent comments have all previous message ids in a References header. No threading.
I tried the same in Gmail, and nothing seemed to thread the messages together. Googling around, it seemed others had come to the conclusion that only the subject line matters. Apparently if two messages have the same subject (plus or minus some "Re:" prefixes), then they are in the same thread.
But what is the actual algorithm? I know that there can be differences in the subject lines ("Re:" and all). What are these mail clients doing to decide that two messages are in a thread?
I like having the author name in the subject line, it makes the Inbox listing richer. But it's also what's keeping these messages from threading. Is there a way to get the best of both worlds?
I know I've seen threads in Thunderbird where the subject line changes completely mid-thread. Is that because they have Reply-To headers? Comment notifications aren't replies to each other, but maybe that's a way to force threading?