Mail on the iPhone mysteriously does not support landscape mode for reading and writing email. Fortunately, an intrepid developer, Mark Schneider, has come to the rescue with an App called TouchType
TouchType costs $.99 and it is a new iPhone App that solves this problem. Open the App and you get a landscape mode keyboard. You can then type your email and hit the send button and it auto-populates the Mobile Mail App. Enter your email address and off goes the message you just typed.
If you would prefer to type your email messages in landscape vs. portrait mode than this app comes highly recommended.