This addon adds a blinded carbon copy (BCC) email address to each outgoing email created by Newsletter. Yes, it means the BCC email address will receive a copy of every sent email, so if you’re sending a campaign to 10,000 contacts, the BCC address will receive 10,000 emails. Be careful!
The addon can be found on GitHub and was implemented for a customer to rotate a copy of each email to their CRM. And it is a very good example of how to code a plugin that deals with messages generated by Newsletter.
This add-on can be cloned, renamed, and used to manipulate every aspect of a message being sent: from the content to the sender, from the headers to the subject.