There is no Inadvertent Disclosure rule found in the California Rules of Professional Conduct. In the absence of California authority, courts have looked to the ABA Model Rules, and the ABA Formal Opinions interpreting them, as well as the ethics opinions of other jurisdictions or bar associations for guidance. (City & County of San Francisco v. Cobra Solutions, Inc. (2006) 38 Cal. 4th 839, 852; Rule 1-100(A) of the Rules of Professional Conduct of the State Bar of California.)
ABA Model Rule 4.4(b) addresses the inadvertent disclosure issue and states:
(b) A lawyer who receives a document
or electronically stored information
relating to the representation of the
lawyer’s client and knows or reasonably
should know that the document or
electronically stored information was
inadvertently sent shall promptly notify
Thus, the receiving attorney is not required to return the inadvertently disclosed material. Whether the attorney should return the email and/or attachments is a matter left to the attorney’s judgment. (See ABA Model Rules Prof. Conduct, Rule 4.4, Comment  (“Some lawyers may choose to return a document unread, for example, when the lawyer learns before receiving the document that it was inadvertently sent to the wrong address. Where a lawyer is not required by applicable law to do so, the decision to voluntarily return such a document is a matter of professional judgment ordinarily reserved to the lawyer.”).) Thus, absent a court order, an attorney is not under a legal obligation to return inadvertently disclosed material. Of course, the receiving attorney should consider agreeing not to review any of the potentially privileged material until a court can rule on the matter.
* Please note that laws can change. These blog posts do not constitute legal advice.