Mysql is used as the database for the email storage. Combine with master to master copy of Mysql a redundancy email server can be setup for backup and disaster recovery.
More detail about Dbmail can found at there website.
From Dbmail Website.
DBMail is made up of several components. A normal MTA (Postfix, SendMail, QMail, Exim) is used for accepting messages. The MTA hands the messages over to dbmail-smtp, using a pipe interface, or dbmail-lmtpd, using LMTP (Local Mail Transport Protocol). These programs take care of delivering the message into the database. Messages can be retreived from the database using dbmail-pop3d, using the POP3 protocol, and dbmail-imapd, using the IMAP4Rev1 protocol.
The whole email is stored in the database. That includes attachments. The DBMail programs do not have to touch the filesystem to retreive or insert emails. User information is also stored in the database, so users do not need an account on the machines DBMail is running on.