Хоумпага http://trojita.flaska.net/
Фичи:
A pure Qt5 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
Support for bandwidth-saving mode aimed at mobile users with expensive connection
OpenPGP and S/MIME cryptography
Safe and robust dealing with HTML mail
IMAP over SSH -- in addition to usual SSL/TLS connections, the server could be accessed via SSH
Версия для Windows https://sourceforge.net/projects/trojita/files/win/