Assume that your employer places responsibility on you for complex negotiations that frequently involve email exchanges of draft transaction documents. One party would do a "redline" copy of the draft submitted by the other party, so that the receiver could see what changes the sender required. On one occasion, your counterpart on the other side mistakenly sent back it's redline to you so that you were able to see text that was not intended for you. Comment boxes and other evidence of what company's employees were saying to one another about their negotiating positions were included. This unintended transparency presumably occurred because the writers did not understand how to use Microsoft Word's track changes function. Using this knowledge would allow you to strike a better deal for your employer. Would it be unethical to use this material?
In my opinion, it would be unethical to use this material, which was unintentionally sent to you. It is a common practice, in fact advisable, to find out as much information as possible about the other party before entering and during negotiations to ensure the best deal for your company or client.
However, in this case, you are privy to the information that was not meant for you. Also, since only some information is available to you, it may not provide you a complete picture of what the other party is thinking or planning, and hence only a part of the picture. In either case, you should return the email to the sender with a note that you will not be using any of the information contained in the email for your negotiations. This is the ethical position. The other party may change their position, given that you have the information, however ethics demand that this information is not used.