Enigmail has been around for a while. This extension allows you to make use of OpenPGP encryption and authentication for your email. Enigmail features automatic encryption/decryption, and key management. The one caveat to this extension is that you must install GnuPG separately (the extension does not do this for you). Enigmail is simple to use; just a click from a drop-down and you can encrypt or decrypt and email. Before using this extension, however, it's best that you have a fundamental understanding of encryption. This extension supports Thunderbird versions 17-19 and can be used on Linux, Mac, and Windows.
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