In Safari, you have the means to switch to a landscape keyboard by simply flipping your device horizontally. The accelerometer will automatically convert your screen. Why this wasn't done for email messaging has been a question that many iPod touch and iPhone users have wondered. Well, it seems like there are a few 3rd party apps found on the App Store to solve this problem. Here's one that's rated the highest by MacNewsWorld.
The Wide Email application from LizzardWerks has the most features for just $0.99. If you have trouble typing on the tiny keyboard when your iPod touch or iPhone is vertical, you can now use the Wide Email application to type everything in landscape mode. This application brings up a screen with a landscape keyboard where you can easily compose new messages or reply to your email messages.
There are other free versions out there, but not with the same type of features that Wide Email provides (Typing Shortcuts, multiple signatures, etc). You can see the entire review here.