It's only ethical to include metadata...

The New York State Bar Association was recently asked the following: "DR 4-101(B) states that a lawyer shall not "knowingly" reveal a confidence or secret of a client. Does a lawyer who transmits documents that contain 'metadata' reflecting client confidences or secrets violate DR 4-101(B)?"

After discussing the nature of metadata and the lawyer's duty to protect client secrets, the Bar Association concluded that the answer was "yes." Lawyers have a duty under DR 4-101 to use reasonable care when transmitting documents by e-mail to prevent the disclosure of metadata containing client confidences or secrets.

Just thought you'd like to know...
The whole opinion is here.