DBMail - fast and scalable sql based mail services
DBMail is an open-source project that enables storage of mail messages in a relational database. Currently MySQL, PostgreSQL and SQLite can be used as storage backends.
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-deliver, using a pipe interface, or to dbmail-lmtpd, using LMTP (Local Mail Transport Protocol).
These programs take care of injecting the message into the database. Messages can be retreived from the database using dbmail-pop3d, using the POP3 protocol, or dbmail-imapd, using the IMAP4Rev1 protocol.
Emails are stored in the database. This includes attachments. User information is also stored in the database, so users do not need a system account on machines DBMail is running on.