Everybody says that the email is a cold way to communicate. I disagree.
We will not forget the limitations of email, but there are many resources to transmit emotions through an email. I am going to focus on the text:
-Orthographic symbols are enough to express feelings: exclamation marks, question marks, 
suspension points ... Everything communicates.
-Bold, italics and underlined: a classical. Emphasize on what interests you.
-Words you use in your email will say the tone: distended or formal. Choose it looking at your target and your organization.
-Phrases. Short one, build a dynamic text; long one,a formal text. All depends on what you want to say.
-If you finish your sentences with a "smiley" will give feelings to the language, expressing your mood. Be careful! Do not fill your email with happy and sad "little faces" Remember, less is more
Analyze how you are doing el-communications with your customers. Do not think you can put something of life in your emails? ;-)