Why doesn't Mail.app properly thread Microsoft Outlook replies?

Question

I use Mail.app 3.6 (on 10.5 Leopard), and I always use plain-text email. Normally, when I receive an email reply from practically anyone, it looks like this (test message sent from Mail.app, replied from Hotmail, replied from Google Apps):

Needless to say, I quite like this threading, and it keeps everything very visually-organized. However, when I receive a plain-text email reply from people on Microsoft Outlook (tested with Outlook 2003 & 2007), it isn't threaded like the image. Instead, it appears as below, without being threaded nicely.

My reply to the message.

-----Original Message-----
From: Tyson Moore
[mailto:[e-mail]]
Sent: [Date]
To: [Receiver]
Subject: Test

Original message.

Through looking at the source of my original message, it appears as though every time the message is quoted, less-than signs (<) are inserted before every line in the reply. I am assuming this is what triggers the quoting behaviour exhibited by Mail.app, but I'm no expert.

My question: is this a Mail.app limitation in not recognizing the -----Original Message----- line put in by Outlook, or an Outlook problem not inserting > before every line in a reply (or both)?

Answer 1

Given the lack of colors within each part, the first layout seems to be the rendering of the text version. Unless you include some formatting, Apple's Mail sends messages using format=flowed text, leaving it up to the recipient to render it in the font and style that recipient likes. (Thumbs up for that!) When replying, it keeps everything plain text (unless you add some formatting). But when forwarding it will include a HTML version with Apple's idea of quoting as well...

For text email, Mail shows > as a single left border, >> with an additional left border in another color, and so on. For the very same message, another email client might really show the >.

If you want a technical explanation for the Outlook message, then we need more details. (Press Option-Command-U to see the actual message.)

Like: probably Outlook sends the message as a multipart/alternative, probably including both HTML^† and plain text. If Mail then shows the HTML version, you might be looking at the formatting Outlook (and hence the sender) gives you. If so, then changing your Mail preferences to show the text version (press Option-Command-P), might give you the layout you want.

^{† Should better have been text/enriched (not HTML), but well.}

Answer 2

It completely depends on the format the program uses... And it so happens to be that Outlook and Mail.app both use different formatting hence only Outlook detects outlook threading and Mail.app doesn't.