Securebird is an unofficial branch of Mozilla Thunderbird, which is optimized for security and speed. Build for OS X 10.11, but works nicely with 10.9 and up.
Securebird uses the same source code like Thunderbird 43. But adds some extras, which makes it more secure and fast.
Securebird is compiled with the hardening flags '-fstack-protector-strong' and '-D_FORTIFY_SOURCE=2', which protects the programm against buffer overflows. For better privacy some prefs are changed and additional patches are included. It includes per default the OpenPGP interface Enigmail extension.
All builds are unofficial versions of Thunderbird. They are optimized for the Intel Core 2 architecture. At least a Mac with a Core 2 Duo and OS X 10.9 is required to use Securebird. The Lightning Calendar is not included. Known issue: Under HiDPI no drag image is shown for drag & drop actions (Bug 933302).