Suppose a political campaign group sent another newsletter by CC’ing their own subscribers’ email addresses.

Each of them will get the message just fine, but they’ll also be able to see the email addresses of everyone else who subscribed.

Not only is this a breach of trust, it can open the door to spammers who get their hands on the list.

Since this is a political organisation, their list of subscribers also reveals their subscriber’s political beliefs – which is classed as special category data.