Vast volumes of email - Intertwingle
An article from Jamie Zawinski, dated May, 1998, part of the
mozilla project. It outlines various ideas such as the inadequacy of
mail folders to categorize and retrieve messages, the need for a
database containing the mail "meta-data", and provides some insight
into what kind of search the author believes would be desired within
that data.
Decimail, an imap server that uses PostgreSQL storage. The main idea behind decimail is to map IMAP mailboxes to user-defined SQL queries.
DBMail, a pop3/imap server that uses PostgreSQL or MySQL storage.
SQmaiL, a Gnome mail user agent with MySQL storage, written in Python. Among the outlined features are SQL-based virtual folders and the ability to handle large mailboxes. But it looks like the project hasn't been updated since an alpha version released in May, 2001.
Zoe ("Googling Your Email")
A web based email client and server said to be remarkable for its
search capabilities and intuitive user interface. It happens to use Lucene, the java
full-text search engine of the
Apache Jakarta Project.
catchmail, a Python script designed to writes emails in a relational database.
Oryx Mailstore is a SMTP and IMAP server that uses PostgreSQL for storage.