This is a very nice treatment of this issue from my friend Nicole Black— Should you copy clients on emails to opposing counsel? Those who have read my musings on the subject know I think BCC should never been used for email, unless you are using that technique to file the emails within your system, i.e. you are BBC’ing internally within the law office.
But is it appropriate to Reply All when the opposing counsel emails you and CC’s his client? That’s a more nuanced discussion and requires analysis. While there are some obvious cases where it is most likely appropriate (e.g. “3:30 p.m. instead of 3:00 p.m. is fine with me), this post suggests it is often not.