If your Lecturer uses a Participation or Online User block in your course other students that are enrolled may be able to see your email address. If you do not want them to be able to, you can edit your profile to "hide" it.
Choose My profile settings > Edit profile
Alter Email display to "Hide my email address...".
Scroll to the bottom and [Save changes]
Do be aware this may make it harder for your lecturer to contact you.