I realize your competitor made a mistake, but I wouldn't send out unsolicited emails. Those people didn't sign up to your list.
If you use email in your business? The CAN-SPAM Act, is a law that sets the rules for commercial email, establishes requirements for commercial messages, gives recipients the right to have you stop emailing them, and spells out tough penalties for violations. I think you are free to use the emails provided you comply with the CAN-SPAM Act. you can read about the act at business.ftc.gov/documents/bus61-can-spam-act-compliance-guide-business
A company's customer list is one of its most valuable assets and courts will go out of their way to protect that company's property right. Just because they made a mistake in sending you the list does not entitle you to either use it or keep a copy. Here's the first analogy that comes to my mind. You pay for your $19 worth of gas with a 20 dollar bill but the cashier gives you change for a 50 dollar bill. Just because the cashier made a mistake does not mean you are entitled to the extra $30 you got in change. If at the end of the day the cashier figured out their mistake and called you to refund the difference you would be required to do so.
I guess the short answer is they can't prevent you from using it. The rest is all about ethics and consequences I think.
I think that when the sent you the email, even they wanted or not, the given you by with thery know, so I guess tat you can use them without any problem.
I think you can likely get away with using it, free of consequences, but I don't know if I would do so. I'm all for shortcuts, but this seems like dirty pool to me. Reverse the roles and think how you'd feel.
I would think that a company would take action against you if you did decided to use it. As someone pointed out: a mailing list is one of the most important tools that a company holds. I suppose it is one of those "ethical" questions.You might also consider the consequences. In the end will the benefits you can possibly receive outweigh the greater forces of the potential legal repercussions.