I generally encourage questions by email. Some questions, of course, are too complicated to be answered by email. In those cases, I will suggest you come by office hours to discuss them.
When sending email to professors (and most other business contacts), however, there are a couple of rules of etiquette that you should follow.
Clearly identify yourself: Include first and last names, and when sending to a professor, tell us what class you're taking. Remember that we have up to 100 students per semester and it takes awhile to remember who is in which class.
Send the main body of your mail in PLAIN TEXT: Sending the main text in non-standard formats as attachments such as HTML, word, PS, etc is risky. Some people do not use web browsers to read email, and many people do not use email readers that can handle both MS-word and PS.
Most professors prefer to receive homework assignments in paper form; it saves us the trouble of printing. When you do send files as attachments (for example, submitting a report when you're out of town), do three things:
In your local work environment, everybody might use the same OS and mail reader, in which case you know the configuration. But when you deal with people outside of your company, allow for incompatibilities.
Remember the volume of email that professors get -- on a typical day, I get 60 to 100 email messages. If I don't know who you are, or I have trouble reading your email, don't be surprised if you don't get an answer.
-Dr. Jeff Offutt