Threading Windows and Web-Based E-Mail Clients

This page discusses issues with e-mail threading on Microsoft Windows operating systems. Threading is an advanced and very, very nice feature in some e-mail clients whereby subjects are grouped together based on who replied to what e-mail messages. For example:

Mar 17 Person 1         PC buying advice
Mar 17 Person 2         |->
Mar 18 Person 3         | `->
Mar 18 Person 4         |   `->
Mar 18 Person 6         |->
Mar 18 Person 6         | `->
Mar 18 Person 4         |   `->
Mar 18 Person 5         |     |->Gaming Machines (was: PC buying advice)
Mar 18 Person 3         |     `->Re: PC buying advice
Mar 18 Person 4         |       `->
Mar 18 Person 7         `*>

As you can hopefully see in that dreadful ascii art (most threading e-mail clients are much nicer looking), it is very clear who responded to who, including when they changed the subject line.

See that last line with "*" in it? That's a broken thread. It means (generally) that that person's e-mail client doesn't respect threading headers (AOL is particularily bad for this) and now we have no idea who in that subject's thread that person was responding to without actually opening the mail. This is very annoying for those of us who are used to this sort of mail reader.

There are two basic issues that need to be kept in mind with threading: if you want to see threads, your e-mail client has to support that, and if you want to not break threading for other people, your e-mail client has to comply to the relevant standards.

Threading Compliance

A truly amazing number of e-mail clients, especially on Windows operating systems, do not preserve threading information in the correct way. To get a nice listing like the above, that works even when the subject line is changed, the MUA (that's Mail User Agent, or mail client software) needs to properly fill the References and/or In-Reply-To field when you reply to an e-mail. This information is then used by a threading mail reader to generate the nice pretty threads.

Hugo Haas has put together an excellent page of non-compliant MUAs, i.e. mail clients that break threading for everyone else. It is somewhat out of date, however. For example, he mentions Outlook Express version 5.50.4133.2400, whereas my machine has 6.00.2800.1106, and I have no reason to believe that that is the latest.

So, here's a list of Windows mail clients that, as far as I know, correctly send out threading headers. I haven't tested any of this, as I don't actually read e-mail on my Windows machine.

Non-Threading Windows And Web-Based E-mail Clients That Preserve Threads

This is far from an exhaustive list; please let me know if you are aware of other clients that should be on this list.

Threading Support

The next step up is clients that actually display threads. Here is the list:

Again, this list is probably not complete; please let me know if you are aware of other clients that should be on this list.