Does anyone know where can I find information about UNIX mailbox files' organization? I am willing to develop a file-mailbox-compliant mail agent, but for doing so I need that information.
It depends on which application is being used to store mail...
You could look in /var/spool/mail/(you), or where ever your mail spools to for an idea of incoming format, but storage-wise, I don't think there's really a standard...