[NT!]

your email is a great way to escalate the situation and make enemies.

first you need to make sure that it doesn't say anywhere in your lease that you are responsible for the area surrounding your walkway or whatever. if that's the case:

To Whom it May Concern,

We received a bill from your company today due to 'trash in public areas.' We did not place any trash in a public area, nor did any of our guests. Therefore we are surprised and confused by this bill, as we are not responsible for this problem and it is not covered in our lease. Please rescind this bill immediately and notify us in writing that you have done same.