The characteristics of a good email message from the options provided are:
- It is short and to the point.
- It is polite and respectful.
- It states the unit and lesson number you are referring to.
- It states the subject of the course you are writing about.
Note: "It is written in all caps." is not a characteristic of a good email message, as writing in all caps is generally considered the equivalent of shouting and can be perceived as rude.