Mail.app in 10.4 stores each message in its own file. The first line has an ASCII integer on it, the length in bytes of the message; then comes the message; then an XML "plist" blob with a few key/value pairs like "sender" and "subject". One of those properties is "flags", holding an integer, presumably bits indicating "unread", "replied", etc. But there are a lot of bits set in there -- some otherwise-uncomplicated messages seem to make use of up to 30 bits. What are they?
Update: Please, people, I asked a very straightfoward question. I'm not interested in your guesses. I can guess too. I'm looking for facts. I can also reverse-engineer at least some of it, if I have to. But I'd rather not, if it's actually documented somewhere. Which is why I asked. If you don't know, put your hand down.
Update: An anonymous benefactor has the goods:
|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|
Thank you Mask Man!
Update, 7 Aug 2005: I've posted my Perl code to parse these files: emlx.pl. Using this code you can, e.g., ssh in to your mac and see what messages Mail.app has downloaded while you were away.