What do you think would happen if I threw away 3 months worth of company mail at work?
That doesn't sound good. Why are you throwing away company mail?
There's nothing for me to do
You shouldn't throw away documents at work without permission. If you don't have anything to do, talk to your supervisor. They can assign you some tasks or reevaluate your workload.
Pass. I'm going to throw away all that mail anyway.
If you throw away that mail and someone finds out, there's a high likelihood you will be fired. I don't see what positive could come from throwing it out, but the consequences of doing it are severe. If you really have nothing to do at work, it would be better just to sit there on your phone instead of throwing away company mail.