Look at the security settings on your webmail account.
Gmail allows you to encrypt all of your mail, but this option is turned off by default. You have to go into "Settings," and then "Browser connection" under the General tab. From there, click "Always use https."
Hotmail and Yahoo mail don't offer the same kind of encryption for email messages, but they do offer it to encrypt your user name and password. In Hotmail, you need to turn this on - click the button that says "use enhanced security" at login. For Yahoo, this feature is always on. If you use a webmail account other than the three mentioned her, check its security settings to see if encryption is available.

See also:

How private are my emails? and What is the best way to prevent phishing?