In a recent Google Official Blog post, Staying at the Forefront of Email Security and Reliability: HTTPS-only and 99.978 Percent Availability, Google announces that

Starting today, Gmail will always use an encrypted HTTPS connection when you check or send email. Gmail has supported HTTPS since the day it launched, and in 2010 we made HTTPS the default. Today’s change means that no one can listen in on your messages as they go back and forth between you and Gmail’s servers—no matter if you’re using public WiFi or logging in from your computer, phone or tablet.

Additionally, all emails sent or received are encrypted internally ensuring that messages are safe even when moving between Google’s data centers. Google further announces that these security enhancements do not affect the availability of the services since in 2013 Gmail was available 99.978 percent of the time.

Additional security email encryption include:

• SafeGmail – free extension for Google Chrome allowing to send encrypted emails
• SecureGmail – Chrome extension encrypting Gmail messages
• Encrypted Communication – free Firefox add-on encrypting messages to be transmitted
• Enlocked – using Pretty Good Privacy (PGP) that comes as a pugin for Firefox, Chrome, Internet Explorer, Safari, and Outlook
• Sendinc – Outlook add-in allowing to send and receive encrypted emails
• Mailvelope – Open PGP encryption for Webmail integrated into the webmail interface and available for Gmail, Yahoo, Outlook, and GMX