I just accidentally discovered an easy way (the easiest way) to delete an email message while using an iPhone.
While you're looking at a list of email messages in a folder on your iPhone -- such as your Inbox -- and you want to delete just one message, just slide your finger across the message in either a left-to-right or right-to-left gesture ("swipe it across the message"). This gesture brings up a red Delete button on the right side of the message. To delete the message, just press that button. If you don't want to delete the message, just press some other part of the message, or swipe the message again.