Suppose that company headquarters has initiated Casual Friday rule at workplace. Rule works fine in most departments, but in the department you have two problems. First dissimilar most departments, you deal with public frequently and unexpectedly; customer can come by anytime. Second, the staff tends to exceed. They wear sandals, shorts and... Well, let us say you turn away your eyes frequently.
.Suppose you have authority to order your staff to do what you wish them to do. Prepare the email in directive mode.
Now suppose that you don't have authority over this group. It is a headquarters policy. Though, you wish to persuade them to dress differently on Casual Fridays. Prepare another email message, this time in persuasive mode.
Don't offer bribe or reward. Encourage your reader by appealing to his or her interests, requirements and desires. Utilize logic and reasoning.
Don't make threats. You cannot say, "Failure to fulfil will result in dismissal." Keep in mind, you do not have authority to do anything. You are relying completely on your persuasive powers.