When postfix delivers mail, something (I assume postfix, maybe the IMAP server) places it in Maildir/new. The file at that point has a system specific file name.
When courier is invoked on an account, it moves all messages from new/ to cur/, and appends :2, to the end of it. This happens before the client even sees the message.
As the client goes through the messages, the file names have additional info tags appended (in alpabetical order) after the comma. So, if a comma is the final character in the file name, the user has not seen it yet. Following codes are appended after the comma to indicate the state:
D - Draft - the user is creating a new message, and it is being edited
R - Replied - The user has replied to this message
S - Seen - The user has "seen" this message (depends on the IMAP client)
T - Trashed - The user has marked the item to be deleted, but has not yet purged the system
F - Flagged - The user has marked the message for some purpose (ie, importance, etc...)
This is message 1352667584.M676313P20021 on server mail.example.com , the user mail reader has seen the message (because it has the :2, on it and the user has read it (the S at the end)
This message has been Seen and Replied to, and something has added anotehr flag to it (in this case, it was Thunderbird marking it with a star).
This message has not been processed (moved into the cur folder) by Courier yet, so it has no :2, at the end. This can only occur if the message is still in the new/ folder.
Taken from http://www.courier-mta.org/maildir.html.