Here is a standard one:
This electronic communication originates from Lawyer A&B, LLP. This
communication and any file transmitted with it contain confidential and
protected information, which may be subject to the attorney-client
privilege, or otherwise be protected against unauthorized use. This
communication is also subject to the Electronic Communications Privacy Act,
18 U.S.C. Sec. 2510-2521. The information herein and any file transmitted
with it is transmitted in this form based on a reasonable expectation of
privacy. See ABA Formal Opinion No. 99-413. If you are not the intended
recipient of this communication, you are hereby notified that any
retention, dissemination, distribution or copying is strictly prohibited.
If you have received this message in error, please advise the sender by
immediate reply and delete the original message.
Emphasis added. There is nothing there about reading the message - that is assumed as there would not be a reasonable way to determine you are not the intended recipient.
And here you can read about what happens when you keep credit card statements sent to you when you are not the intended recipient.