To secure your email effectively, you should encrypt three things: the connection from your email provider; your actual email messages; and your stored, cached, or archived email messages.

If you leave the connection from your email provider to your computer or other device unencrypted while you check or send email messages, other users on your network can easily capture your email login credentials and any messages you send or receive. This hazard typically arises when you use a public network (the Wi-Fi hotspot in a coffee shop, say), but an unencrypted connection can also be pose problems on your work or private network.

Your actual email messages are vulnerable as they travel over the Internet, after leaving your email provider's server. Bad guys can intercept a message as it bounces from server to server on the Internet. Encrypting your messages before sending them renders them unreadable from the point at which they embark on their journey to the point at which the intended recipient opens them.

If you leave your saved or backed-up email messages (from an email client program like Microsoft Outlook) on your computer or mobile device, a thief or snoop might be able to gain access to them, even if you've password-protected your email program and your Windows account or mobile device. Again, encryption renders them unreadable to the intruder.