In 2007 somebody wrote a paper about it. Not about therapy, but about making copies of private letters and to whom did they belong. I think he would say you could print off the email and bring it to session to discuss, but not forward it or in any other way make a copy of it for your t. You could probably bring it up on your phone for your t to read, or sign on to your t's computer at their office. In the latter case, maybe a case could be made that a copy then exists but that would be cutting it pretty fine I think. It's the content that is copywrighted to person b.

ETA: whenever my mother writes me a letter, I bring it straight to my t's office. I'm not going to ask her permission.