Enigmail enables Mozilla Thunderbird users to use OpenPGP (using GnuPG). While this should be an integrated part of any email program today, Enigmail strives to give users the best possible user experience an AddOn can do.
Enigmail does what it promises. It simplifies considerably a process which should occupy your brain for some time under normal circumstances. This and its reliability make it a rather good software. But all this ease of use and your success in using encryption, should rather augment your interest in the topic, stimulate your curiosity and encourage you to get a little background-knowledge. Please do not ask, how secure is it? Rather get the information from any GnuPG//OpenPGP-related documentation. If this thing is too simple, you become the risk.
excelent add-on for thunderbird/icedove and must for sending encrypted email