Mailpile

Mailpile is a modern, fast e-mail client with user-friendly encryption and privacy features. Powerful search and tagging make your pile of mail easily manageable.

The program is designed to be fast and responsive, outperforming "the cloud" even on slow computers. Your search index is encrypted so if your computer gets stolen, your mail stays secure. OpenPGP signatures and encryption are part of Mailpile's core design.