Usability 
Email is not a secure service. Even if you use a secure transport like HTTPS or SSL to access your mail, once you messages leave your provider's mail server they travel unencrypted over the Internet to the recipient's mail server. For this reason you really should send credit card numbers, SSNs or confidential personal or business information in regular email. Messages can be intercepted and read by employers, ISPs, government agencies and hackers. Fax or even a voice phone call is significantly safer but still subject to eavesdropping particularly by telephone service providers and the government.
Secure email is possible only if messages are encrypted by the sender and decrypted by the recipient using something like OpenPGP, BouncyCastle or GNU Privacy Guard. These tools are somewhat cumbersome to use and require both sender and recipient to have the encryption package installed on their PC. These tools are also not available for most mobile phones. However there is an alternative that's relatively easy to use, secure web mail. The oldest (since 1997) and largest secure web mail provider Hushmail.com, recently launched a mobile web based version of their product at m.hushmail.com.
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