Online instructors use email as one of their primary means of communication with students. However, some email clients may see your emails as spam so your students may miss getting them because they don't remember to look in their spam folder.
Deanna Thompson created a tutorial for students that shows them how to put you on their safe senders list so your emails won't go to the spam folder. You can give your students this link - http://www.cuyamaca.edu/helpdesk/tutorial_email.asp - to view the tutorial if they are having problems receiving your emails. You might want to put the link in your syllabus or your initial email to students each semester.