MB-IMAP is a GPL'ed IMAP4rev1 server written in C that uses a Relational Database (RDBMS) like PostgreSQL, SAPDB, ... for message storage.
It makes heavy usage of Stored Procedures which makes the server cleaner because most stuff is done by those procedures whereas the server itself does not much more than calling them with the right parameters.

Right now, the primary database target is PostgreSQL (most stored procedures are already written) using its PL/PGSQL

My secondary database target will be SAP-DB (full blown commercial SQL-Database, now GPL'ed).