"Do not mail"?

[Lindsey Johnson]

Hey all, question for you. 

I recently pulled and cleaned a list to direct mail that I really liked.. Great topography and congruent pricing. However, when I went to clean the list, I noticed that every lead was marked "do not mail". This seemed strange so I looked at my other lists and found that generally, only 1-2/1000 is marked "do not mail". So.. wondering.. 1. Do you all mail do not mails? Is it a mater of morality or legality? 2. Have you ever seen a county, city, or zip as a whole prohibit mailing? Any insight is helpful. 

Thank you!