The ABA Journal published today an article concerning an assistant prosecutor's faux pax when she hit "Reply All" when making a comment about a message that the defendant in a criminal case was too ill to attend her trial. Commenting to a colleague, she typed "Surprise, surprise." Although she immediately perceived that her comment might be interpreted as insensitive and tried to recall it, the recall failed. The offender drew a suspension without pay for 3 days and was taken off the team trying the case.
What does that have to do with protecting a client's confidentiality?
I never (and I mean never, ever) blind copy a client on email correspondence to opposing counsel. The danger is too high that a client may accidentally hit "Reply All" when commenting on the content of the email. If that happens, then the client's confidential attorney-client communication will be sent to opposing counsel. This could result in a significant disclosure of strategy.
Instead, I send an email to opposing counsel and then I go to my "Sent folder" and forward it to the client. That keeps my client in the loop, but eliminates to possibility that any response from the client might be sent to OC.