Rainloop: A free webmail application which works without database
Rainloop is a free webmail application written in php, it is free and open source, its source code you can get from sourceforge.net and github.com.
Rainloop has awesome user interface and work perfectly without any database connectivity besides non database connectivity it supports both SMTP and IMAP to easily send and receive emails without any problems.
Due to no connectivity of database Rainloop does not store anything as it only display emails, when you compose your emails it connects with your actual/linked mail server and send/receive emails via mail server only.
Rainloop support popular social websites i.e. google, facebook and twitter so that you can directly link your accounts to this mail app to send and receive emails, it also support multiple accounts to send/receive your mails from multiple email accounts hassle-free.
It has beautiful modern user interface and anybody will love to work on.