Trojitá is a very fast and lightweight IMAP e-mail client written using the Qt library. While it is not intended for use as a regular e-mail client just yet, it could be interesting to play with it to see if the speed looks compelling to you.

What could make Trojitá interesting for you:

A pure Qt4 application with no additional dependencies; it builds in two-and-half minutes on a five-year-old laptop
Robust IMAP core implemented using Qt's Model-View framework
Standards compliance is a design goal
On-demand message list and body part loading ("lazy loading")
Offline IMAP support
Support for bandwidth-saving mode aimed at mobile users with expensive connection
IMAP over SSH -- instead of going over an SSL socket, the server could be accessed via SSH
Safe and robust dealing with HTML mail