While many individuals and business use a non-disclosure footer at the bottom of their emails, the fact is you can't make an agreement with someone who hasn't actually agreed to it. In other words, an email isn't like going to the car wash where it may be posted "we're not responsible for damage to your vehicle". There you agreed if you decided to continue to use the car wash. An email if mistakenly sent to the wrong person, or if forwarded to someone else just because you have a disclaimer at the bottom is very much different. Neither party has agreed to anything. It's more of a scare tactic than law.
Here's a great article from the American Bar Association
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Please let me know if you have any additional questions.