Trojita is a "fast" IMAP e-mail client that's wirtten with a Qt interface. The open-source program's mission is to have mail access anytime and anywhere, does not slow down, respects open standards, is efficient, works on multiple platforms, and plays well with the rest of the software ecosystem.
It was recently decided that Trojita should be part of the KDE project, and so it's been agreed upon to fold it in. This change was announced from a blog post. While now part of KDE, Trojita will still aim to be cross-platform and not dependent upon KDE-specific libraries.
For anyone now just becoming familiar with the Trojita IMAP email client, there's more details and download links from the project site. With Trojita being only IMAP-focused and concerned just about performance, it's unlikely to unseat KMail/Kontact from the KDE world as the default email application anytime soon.