Email protocols (SMTP, IMAP and POP3) are not secure protocols. Usernames, passwords and email contents are sent "in the clear" over the internet, making them susceptible to traffic snooping. There have been many instances of passwords being snooped over unsecured wireless networks and used to send malicious emails using legitimate credentials. Using a secure protocol, you can send your password and emails in a secure manner so that they cannot be snooped.
Each protocol has a secure equivalent:
SMTPS - For sending emails
IMAPS - For viewing mail on the server
POP3S - For receiving email
All of these encrypt passwords and data transferred to/from the server.
Enabling Secure Email
Each email client is different. You should look for settings that mention the secure protocols above. Often mail clients have an option that mentions terms like "SSL" or "TLS". These should be selected or checked to enable secure email. You should look for the following options:
Secure email protocols use SSL certificates to authenticate and verify the server. This certificate is issued in the server name. If you connect using your own domain name (e.g. mail.yourdomain.com) or an IP address (e.g. 188.8.131.52) then you will probably get a certificate warning message. To avoid this, use the server name (e.g. legolas.z-host.com) instead. You can find the name of the server hosting your site on the domain details page.
Email Client Configuration
Outlook 2003, Outlook 2010:
View your email account properties. On the "Internet e-mail settings" panel, click the "More Settings..." button. On the next dialog click the "Advanced" tab. Enable both checkboxes for "This server requires an encrypted connection (SSL)".
Windows Live Mail:
View your email account properties. On the "Advanced" tab, enable both checkboxes for "This server requires a secure connection (SSL)".
Use Secure Connection: SSL Encryption
From the Preferences page, select the "Accounts" tab. From the "Outgoing Mail Server (SMTP)" option, choose "Edit SMTP Server List". Click on the "Advanced" tab, tick the box "Use Secure Sockets Layer (SSL)". Repeat these steps for the "Incoming Mail Server (IMAP)" option.