You can send bulk messages from the Case search.
Choose the cases you want to send the message out to and then hit the bulk message
A window for the message will appear:
Once you confirm the sending, you will be directed to the overview page, where you can see the progress of the sending as well as the previous bulk messages
Under the details button you can find information about the sent message:
Don't see the option to send bulk messages? Reach out to your account manager :)