Jaxbot over at Windowsphonehacker has released a new homebrew app for Windows Phone 7 devices. It is a simple app called EmailKey, you can type on your email address and press Set To @. Once you have set, the application will add your email address as an alternate mapping for the @ button. So, whenever you need to enter your email address, you need to hold down @ on your keyboard and your email address will be shown as a suggestion from which you can select it. You can disable this feature whenever you want.
Also using the new WPH Tweaks app, you can customize any button on your keyboard and add any number of suggestions. Download it from the link below.