mbox vs. Maildir

While mbox was the original mail storage system included with WHM and cPanel, Maildir is now the recommended option.

Whereas mbox works by placing all messages in the same file on the server, Maildir works by keeping each message in its own file with a unique name. Maildir is compatible with both the Courier and Dovecot mail servers, is more current, faster, and more stable than mbox.