Mu4e is an emacs-based email client that allows users to handle e-mails (such as searching, reading, replying, moving, deleting) very efficiently. The basic idea is to configure an offline Imap client that allows syncing your local computer with a remote email server.
Entirely search-based without any folders, only queries.
Easy documentation with example configurations.
User-interface designed for speed, with fast keystrokes for common actions.
Support for signing and encryption.
Address auto-completion as per your existing messages.
Extensible with available snippets or with your own code.