SecureGmail encrypts and decrypts emails you send in Gmail. This happens all on your machine, and the unencrypted text never reaches Google servers. This is useful if you don’t want anyone but the intended recipient to ever read your email (i.e. companies, governments, etc.)
How secure is it?
SecureGmail uses symmetric encryption to encrypt and decrypt each message. The password is decided by the user and assumes the recipient already knows it. SecureGmail is only as good as your password, pick an easy to guess password and it will be easy to break. Shared knowledge can be a useful and convenient password. Remember to never email or IM your password for others to intercept.
How it works?
You can also check out how it works in the below video by LiudasBar