At this rate, we're going to have quite a few ethics opinions on metadata over the next couple of years. Here is what Arizona has to say on the topic.
In essence, the Arizona State Bar Association has concluded that senders of electronic documents must take reasonable precautions to prevent the disclosure of privileged metadata. As for recipients, the opinion joins New York, Alabama, and Florida in concluding that, in most cases, lawyers shouldn't rummage through metadata. The opinion says that, except in specific circumstances, such as when an opponent consents or other law or rule (such as a discovery order) allows it, “a lawyer who receives an electronic communication may not examine it for the purpose of discovering the metadata embedded in it.”
If you're keeping score, that's Alabama, Florida, New York, and Arizona saying that we shouldn't normally be allowed to look at metadata. And then there's Maryland, the D.C. Bar, and the ABA saying that it should typically be permissible. Finally, there's Pennsylvania, which has staked out a middle ground approach.