I can't find any documentation on how postfix stores email. Where is it stored, and in what format? I'm using Ubuntu server 11
/var/mail/[username] or the more traditional
The normal format, called "mbox", uses a line that starts with "From " to indicate the start of each message - this is one reason why many email clients will change "From " in the body of the message to ">From ". You can also configure it to use "maildir", in which
/var/mail/[username] is a directory in which every email message is a file in that directory.
For outbound mail (you didn't specify, and inbound is answered elsewhere) it is in
/var/spool/postfix. If you look there you will find all sorts of subdirectories.