Most tutorials on UNIX mail services skip section related to mail directories. However joe's (you can get his book Unix for the Beginning Mage here) tutorial starts with /var/spool/mail directory and it further explains the difference between mbox (an electronic mail file format) and maildir format. It explains fundamentals in joe's own classic style :)
"It was Acre's first day of training in the Towers mail department over at /var/spool/mail. This is where all the mail is stored and waits to be retrieved by the POP and IMAP daemons.
"How do these mailboxes work?" Acre asked the admin on duty.
"Well," the admin replied, "it's really not that complicated. Our Tower users a mail storage format known as maildir."
Read it online.