Colorado has addressed whether lawyers can review metadata sent by opposing counsel outside the context of discovery. The summary states:

A Sending Lawyer who transmits electronic documents or files has a duty to use reasonable care to guard against the disclosure of metadata containing Confidential Information. What constitutes reasonable care will depend on the facts and circumstances. The duty to provide competent representation requires a Sending Lawyer to ensure that he or she is reasonably informed about the types of metadata that may be included in an electronic document or file and the steps that can be taken to remove metadata if necessary. Within a law firm, a supervising lawyer has a duty to ensure that appropriate systems are in place so that the supervising lawyer, any subordinate lawyers, and any nonlawyer assistants are able to control the transmission of metadata.

A Receiving Lawyer who receives electronic documents or files generally may search for and review metadata. If a Receiving Lawyer knows or reasonably should know that the metadata contain or constitute Confidential Information, the Receiving Lawyer should assume that the Confidential Information was transmitted inadvertently, unless the Receiving Lawyer knows that confidentiality has been waived. The Receiving Lawyer must promptly notify the Sending Lawyer. Once the Receiving Lawyer has notified the Sending Lawyer, the lawyers may, as a matter of professionalism, discuss whether a waiver of privilege or confidentiality has occurred. In some instances, the lawyers may be able to agree on how to handle the matter. If this is not possible, then the Sending Lawyer or the Receiving Lawyer may seek a determination from a court or other tribunal as to the proper disposition of the electronic documents or files, based on the substantive law of waiver.

If, before examining metadata in an electronic document or file, the Receiving Lawyer receives notice from the sender that Confidential Information was inadvertently included in metadata in that electronic document or file, the Receiving Lawyer must not examine the metadata and must abide by the sender’s instructions regarding the disposition of the metadata.

The full opinion, No. 119 (May 17, 2008) is here.