If you were to open an email that wasn’t intended for your eyes, and you were to read unflattering and hurtful things about yourself that might not be true, but that the author, a coworker apparently believes, well then you would be in a bind when you realized that you weren’t supposed to read the email in the first place, and thus can’t retaliate.
Sometimes the consequence for unethical snooping is built right into the crime. If you can’t admit you did something, then not being able to act on what you discovered is punishment enough.
But, honestly, if you saw an email with your name in the title, written by a coworker who you know has it in for you, would you be able to resist opening it? Right. I didn’t think so.