The characteristics of a good email message from the options provided are:
A. It is short and to the point.
B. It is polite and respectful.
D. It states the unit and lesson number you are referring to.
E. It states the subject of the course you are writing about.
Option C (It is written in all caps) is not a characteristic of a good email message, as writing in all caps can be perceived as shouting and is generally considered unprofessional.