Kerio MailServer 6.5 uses Firebird
Excerpt from article on ComputerWorld:
Vitek says MailServer 6.5 also includes an updated Outlook plug in that lets people read and write mail, make calendar changes and other groupware tasks while disconnected from the Kerio MailServer. The new plug in includes a Firebird database that captures all the work you do while on the road and updates everything once you connect back to the server. Another benefit of using Firebird, Vitek points out, is that users now will have full-text search of everything stored in the database.