Documentation on Apple Mail's .emlx data structure(s) (for conversion purposes)?

Here is an emlx2mbox converter in ruby: Mailbox Converter.

I don't think it was written from any documentation of the spec, but it has undergone multiple updates, so hopefully evolved to handle at least some of the quirks of the format. The source code is about 250 lines long, and it looks readable and well-commented.

A few more information documenting emlx format.

The message is composed:

• a byte count for the message on the first line
• a MIME dump of the message
• an XML plist

The XML plist contains certains code such as

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
        <key>date-sent</key>
        <real>1362211252</real>
        <key>flags</key>
        <integer>8590195713</integer>
        <key>original-mailbox</key>
        <string>imap://****@127.0.0.1:143/mail/2013/03</string>
        <key>remote-id</key>
        <string>252</string>
        <key>subject</key>
        <string>Re: Foobar</string>
</dict>

The flags have been described by jwz and represents a 30 bit integer:

0      read                      1 << 0
1      deleted                   1 << 1
2      answered                  1 << 2
3      encrypted                 1 << 3
4      flagged                   1 << 4
5      recent                    1 << 5
6      draft                     1 << 6
7      initial (no longer used)  1 << 7
8      forwarded                 1 << 8
9      redirected                1 << 9
10-15  attachment count          3F << 10 (6 bits)
16-22  priority level            7F << 16 (7 bits)
23     signed                    1 << 23
24     is junk                   1 << 24
25     is not junk               1 << 25
26-28  font size delta           7 << 26 (3 bits)
29     junk mail level recorded  1 << 29
30     highlight text in toc     1 << 30
31     (unused)

Sending myself a simple message and removing some details, so you can see the full data structure of emlx files.

875       
X-Spam-Checker-Version: SpamAssassin 3.3.2 (2011-06-06) on ******.*********.***
X-Spam-Level: 
X-Spam-Status: No, score=-3.2 required=4.2 tests=BAYES_00,RP_MATCHES_RCVD,
        SPF_PASS,TVD_SPACE_RATIO autolearn=ham version=3.3.2
Received: from [127.0.0.1] (******.*********.*** [***.**.**.**])
        by ******.*********.*** (8.14.5/8.14.5) with ESMTP id r2TN8m4U099571
        for <****@*********.***>; Fri, 29 Mar 2013 19:08:48 -0400 (EDT)
        (envelope-from ****@*********.***)
Subject: very simple
From: Karl Dubost <****@*********.***>
Content-Type: text/plain; charset=us-ascii
Message-Id: <4E83618E-BB56-404F-8595-87352648ADC7@*********.***>
Date: Fri, 29 Mar 2013 19:09:06 -0400
To: Karl Dubost <****@*********.***>
Content-Transfer-Encoding: 7bit
Mime-Version: 1.0 (Apple Message framework v1283)
X-Mailer: Apple Mail (2.1283)

message Foo
-- 
Karl Dubost
http://www.la-grange.net/karl/
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
        <key>date-sent</key>
        <real>1364598546</real>
        <key>flags</key>
        <integer>8590195713</integer>
        <key>original-mailbox</key>
        <string>imap://********@127.0.0.1:11143/mail/2013/03</string>
        <key>remote-id</key>
        <string>41147</string>
        <key>subject</key>
        <string>very simple</string>
</dict>
</plist>