Send questions and comments to everyone on a list only if many people could be interested in receiving it. Keep it brief, if you can. Several short messages are much better than one very long one. Teachers do not have the time to read through several pages of text. Break your writing into short paragraphs.
Long paragraphs are more difficult to read on a screen.
Never forward or redirect a message sent to you personally to someone else without permission from the author. Don't put anything into an e-mail message that you would not want to see on the evening news. E-mail is about as private as a Fax. It is quite possible that someone else could read it. A spam is defined as sending the same message to a very wide audience.