Easywriter (free app) works well. It is basically a horizontal keyboard for typing emails, meaning you can use both thumbs easily plus there are fewer mistakes.

The best way of using it is to click reply to a mail in the mail programme, then press the home button and go to Easywriter to type the mail, then click send to mail and the text appears in your email you started.

Given Apple won't let you type a mail with a wide keyboard, this is the next best thing.