Mew is a mail-reader on Emacs, which is free to use and whose source code is open.
Users need to configure their e-mail addresses and e-mail servers but such information is not sent anywhere. Users are not trucked at all.
Home page: http://www.mew.org/
Installation: The github version is available via MELPA and the latest stable release is available via MELPA Stable.
Installation by hand: INSTALL.md
Developers: if you want to use Mew cloned from this repository, put the followings to your ".emacs".
(setq mew-icon-directory (expand-file-name "SHOMEWHERE/mew/etc"))
(add-to-list 'image-load-path (expand-file-name "SHOMEWHERE/mew/etc"))