Don't know if this is a bug or a feature...

Replication steps:

• Create a course C
• Create a forum activity in the course
• Enrol a student S and a teacher T into the course
• Make sure that student S has subscribed to forum notifications
• Login as teacher T
• Create a post in the forum -> student S should receive a forum notification mail
• Set the course to be invisible
• Create another post in the forum

Actual Result:
Student S will receive another forum notification mail although the course is invisible and thus inaccessible for him

Expected Result:
Student S should receive no forum notification mail when the course is invisible and thus inaccessible for him