I am regularly asked for guidance when it comes to forwarding emails written by others. Should you ask the sender if it is okay to forward an e-mail they sent privately to you? A resounding YES!
Common courtesy alone would dictate you don’t forward without the original sender’s knowledge.? They may not appreciate the e-mail they wrote to you being sent to others — whether they know them or not. Reverse the roles and how would you feel?
All e-mails are naturally copyright protected by the author upon their creation. To forward a private e-mail to another person, or to post it anywhere online you need to get the author’s permission to do so first.
Cc’ing doesn’t negate asking for permission. E-mails are written for the eyes of the recipient which does not include tacit permission to forward to others without their knowledge or permission. Cc’ing simply advises you are letting them know after the fact that you are forwarding their private e-mail without any courtesy or concern for their feelings.
I have an article on forwarding on my site that you might find helpful that covers the 5 Rules for Forwarding