They can - we just need to create a course specific email address and let people email to that, and determine the poster either by sender email address (they will have to use the same email that they registered with us, and this is also less secure) or embed a user token in the "To:" field (more secure, but harder for people to remember)

For us, when people ask questions, they are likely on our site already, either working on homework assignments or watching lecture videos, so initiating a thread with email is less of a need.