Once mail is accepted by your MTA for local delivery, it does need to know where to store it. Whatever method you use is going to be dependent on the MTA as well as the POP/IMAP daemon that is running.

Nearly all MTA’s and POP/IMAP daemons know how to deliver to mbox. This format simply appends each received message into a file called “inbox”. Out of the box, nearly all MTA’s will come configured this way, and it does have decent performance for a lightly loaded mailserver.

The alternative is called “maildir”. This sets up a directory structure whereby each message that is received is it’s own file. This is how most major mail hosting providers run their infrastructure. The reason being is that when a user checks their email, or the MTA has to deliver a newly received message, it does not have to open up a single file and append/read to/from it. This comes into play when a user has a lot of stored mail on the server.

You will also need to verify that your POP/IMAP daemons can work with the maildir format. Currently the most widely used one is Courier, which is installed when (for instance) somebody running cpanel wishes to switch from mbox to maildir.