This tip will help you to enter the domain suffix portion of e-mail addresses faster in iPhone's Mobile Mail app. (If you've used Mobile Safari with it's handy .com key, this tip may sound familiar.)
In order to quickly select a domain suffix on the Mail app, first press and hold the period key. Mobile Mail then presents you with a pop-up list of domain suffixes to choose from, including .com, .org, .edu, and .net. You may even see some foreign domains--.de for Germany, for example--if you have international keyboards turned on.