Email cluster
The trio Exim4/DBMail/MySQL permits to build highly scalable email services. Data backend is a MySQL database, possibly a MySQL cluster. This allows concurrent access to emails from several SMTP, LMTP, POP3 and IMAP4 servers.
Overloaded mail server do not have to be, clusters of SMTP, LMTP, IMAP4 and POP3 servers provide high throughput, reliability and scalability.

Links:
[1] http://www.exim.org/
[2] http://www.dbmail.org/
[3] http://www.mysql.com/
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