Trojita is a Qt IMAP e-mail client.
Some highlights of Trojitá are:

• A pure Qt4 application with no additional dependencies
• Robust IMAP core implemented using Qt's Model-View framework
• Standards compliance is a design goal
• On-demand message list and body part loading
• Offline IMAP support (you can access data you already have; there's no complete "offline mail access" yet, though)
• Support for bandwidth-saving mode aimed at mobile users with expensive connection
• IMAP over SSH -- in addition to usual SSL/TLS connections, the server could be accessed via SSH
• Safe dealing with HTML mail (actually more robust than Thunderbird's)