Unix mailboxes are flat files. Each message starts with the pattern:

/^From /
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Immediately following the From_ are the message headers, followed by a blank line, followed by the body of the message. The message ends at the next From_ or at EOF.

As Stevie-O suggests, you'll probably want to use a mail parsing module for this. It's not that hard to write your own, but almost nobody gets it right the first time, and it's easier to just use one of the modules