Hi Stefano,
I am building the messaging system for our students in the University and I wondered, how do I output to REST API (JSON) all the newsletters that been sent in this format:
Hi, the "students_emails" attribute is the list of people which received the email?
Anyway you should query and build a serializable structure (array of arrays or objects) getting all email from wp_newsletter_emails table and for students using the sent table.