This is a great subject; unfortunately the PopSci article has nothing to say on the subject beyond what's quoted in the blurb there.
There are two ways to encrypt email (well, for the purposes of this comment, anyway): S/MIME and PGP.
PGP (preferably GPG) is great for encrypting and signing cleartext through media that don't otherwise support encryption -- you can send ASCII-armorered cipher text through email, IM, etc.The PKI is a decentralized web of trust, but it's not particularly easy to use or transparent. Commercial products like PGP blast plugins all over the place to smooth the learning curve. There are probably decent browser extensions to facilitate GPG operations in webmails.
S/MIME is ideal when you're using a dedicated email client that supports it. Based on X.509 certificates (like SSL) and implemented as a feature in mail clients, signing and encrypting messages is as easy as pressing a button or setting a default. Not great for the super-paranoid since most of the complexity is hidden from the end user. The PKI is the familiar trusted-root hierarchy, though self-signed certificates are common.
All cases of email encryption are dependent on secure key exchange and diligence. Historically, it's been tough to get buy-in from interlocutors for message encryption. Even today I doubt many people really care, and the password-protected PDF is still probably the most common message security in use.