Can I use MySQL (or another SQL database) as the primary mail store?

Using a database as the main message store for Cyrus is not currently supported. According to discussions on the mailing list, there is little benefit in adding such support (given typical IMAP access patterns, optimizations in the current mail store that make it fast, and the amount of effort involved to retrofit a different mailstore into the backend), so it’s unlikely to ever be written.

The Cyrus mail store is a normal directory tree, with mailboxes stored as directories and messages stored as individual files. Some additional information, eg index data, is kept in the “cyrus.*” files in the mailboxes.

Cyrus DOES have the option of using databases of various types for storing some other information, such as authentication data, mailbox lists, etc. There may be reasons to add a SQL backend to these databases in the future (in addition to flat, and skiplist).