Advice for Young Lawyers on Email Ethics

Can a lawyer copy another lawyer's client on an email message?  I addressed this question in a column published by the North Carolina Bar Association in The Advocate (the official publication of the Association's now 5,241 young lawyers) on June 28, 2013:

Ask Atticus (An Advice Column for Young Lawyers)

When Sending an E-Mail Message to Opposing Counsel, May I Copy An Opposing Party?

By Matthew A. Cordell

Dear Atticus,

I am negotiating the terms of an agreement on behalf of a client, and the other party’s lawyer routinely copies her client and mine on her e-mail messages. Is this permitted? May I use the “Reply All” function to copy her client on my responses? 

Regards, 

Conflicted in Charlotte

You can read my response here: http://younglawyers.ncbar.org/media/29407140/yldjune2013.pdf#sthash.37d6Dg3t.dpuf