For some strange reason the Apple iPhone doesn’t support landscape mode when reading or writing an email, however that can soon change because an iPhone developer has put out the TouchType application.
The TouchType app for iPhone has been created by iPhone developer Mark Schneider, and allows for a landscape keyboard, you can then type out your email, press send and it then auto-populates the Mobile Mail application, then enter the address and bingo all done.
The TouchType app for iPhone costs $0.99 from the Apple iTunes App Store and requires iPhone 2.0 software update.
Source — iphoneatlas