Enigmail is a security extension to Mozilla 
Thunderbird and 
SeaMonkey. It enables you to write and receive email messages signed and/or encrypted with the 
OpenPGP standard.
Sending and receiving encrypted and digitally signed email is simple using Enigmail.
When starting it for the first time, you are guided through the basic setup. We also prepared a new users guide that explains how to use OpenPGP.
Features List
- Tightly integrated suport for OpenPGP with the Mozilla Thunderbird and Mozilla Seamonkey mail clients
- Encrypt/sign mail when sending, decrypt/authenticate received mail
- Support for inline-PGP (RFC 4880) and PGP/MIME (RFC 3156)
- Per-Identity based encryption and signing defaults
- Per-Recipient rules for automated key selection, and enabling/disabling encryption and signing
- OpenPGP key management user interface
- Automatically encrypt or sign attachments for inline PGP messages
- Powerful GUI for easy configuration and OpenPGP key management
- Detailed user preferences for advanced configuration
- Integrated OpenPGP PhotoID viewer
- Supports OpenPGP key retrieval via proxy servers
- Integrates with GnuPG, version 1.4.x and 2.0.x
- Supports Mozillas Multiple Identities feature
- Available for: Windows / Mac OSX / Linux x86-32 (all in one official release file), and contributed / distribution builds for other operating systems such as Linux, Solaris, *BSD and OS/2)
- Many Language Packs, (mostly) included in the official releases, and available separately for localisation of contributed builds
Comments and Reviews
I really want to like and use GPG/PGP and have made multiple attempts over the years to get it working smoothly. But especially in today's world it may not be tenable for many users. If you have have a single email account which you use on a single device, sure. But when you start adding accounts (even worse: aliases) and multiple devices it can become a mess.
Something I didn't realize prior to spending hours trying (and ultimately failing) to get myself set up with email encryption: the only reason it makes sense to do this is if you plan to email people who also have it set up. Which means your contacts must already know about this, or you are going to have to teach them.
For the very narrow slice of people with appropriate use case, perhaps this is useful, but if you want a pragmatic solution go for an email provider that does the work for you like protonmail or tutanota.
(Hopefully information is accurate; I am obviously not an expert and this is just what I have learned. If someone more qualified tells you something contradictory you should believe them.)