Recently, Blackberry has been in the news because corporate Blackberry’s security is so good that nobody can read emails sent and received by them. Does that mean emails sent or received by consumer Blackberries and iPhones aren’t secure and can be read?
— Royce
When talking about email security, ther are a lot of pieces: the server you are using, your wifi network, the strength of your password, can someone look over your shoulder, etc.
Sending and receiving data over a wireless network should be very secure. Over wifi, it should be just as secure as using a laptop -- with concerns about open wifi networks, etc.
So what is your concern? If you work with high security data, then I'm sure you have someone in your organization that can advise you. If you are just worried about identity theft, then a strong password and avoiding public open wifi is your main concern. See http://macmost.com/online-password-security.html