Hidden blocks and hidden courses:

The idea of iMail, in general, is to facilitate communication between members of courses. Thus, if the a user is enrolled in a course, she/he can see the inbox for that course and email other enrollees of the course, even if the block is not installed in the course, or if the block is hidden in the course.

However, if the course is 'hidden' or set to be not available to students, the students cannot see the inbox for that hidden course.

Metacourses:

When a meta-course is used, students see an inbox for the metacourse as well as the inbox for the child course they are enrolled in (if they are enrolled in a child course). This enables users to email students who are in their same child course specifically, as well as email students who are in the metacourse.