Hi @DorothyMooney,
I will assume you mean a Canvas inbox message (which is similar, but not the same thing as an email). If you have not had a class with the teacher you're looking to reach out to, you may not have the ability to message that teacher. It will depend on the permissions your school/institution has set up for messaging. When composing a message, if you click the "To" box, you can see if you have the option to select "users", which would populate with all of the students and teachers, and you could select from there. In the case that you don't have access to that, you'd probably need to reach out to the teacher in some way outside of Canvas.
Hope this helps a bit!
-Chris
