Yesterday, we added the ability to send email from one record in a many-to-many relationship to all the related records. And example is a Teacher and Student Custom Objects relationship set to many-to-many (each teacher can have many students, and each student can have many teachers). An email configured to go to the related object records, set for the Email address, would be sent to all the related object records.
A teacher could send an email to all the related students, and each student would receive the same email. A student could send an email to all of her related teachers, and each teacher would receive the same email.
When creating the email drill down in the Send email to field to the Related Object >> Related Object Record >> Email. In this example we are drilling down to "Related Contacts" >> "Related Contact" (single record) >> Email field.