The older format is called the “mbox” format. This essentially collects EVERY email to a local user in ONE GIANT FILE. For example, Exim4 defaults to using the “mbox” format. When Exim4 receives email on port 25, it first gathers everything up into the /var/spool/exim4 directory. This directory is not your concern right now! Consider it a temporary playground for Exim4.

Once Exim4 is finished messing around with the email, it appends the email message (and attachments, too) to the file.

/var/spool/mail/john

This is your INBOX (in mbox format)! It is important to remember that the other email folders with which you are probably familiar (like SENT folder, or DRAFTS, or TRASH, etc) have NOTHING to do with your MTA or your INBOX. Those other folders are completely invented for convenience by your particular email *client* (the Mail User Agent - MUA - more on that later), and they are usually stored elsewhere.