Steps to delete a contact from DNM list:-
1 Please hover on the account name (on the right-hand side) and click on account settings.
2 It will divert you to the detail page please look for "Do Not Mail List", it has 3 options kindly click on manage.
3 Now you are on the page where you can see all the contacts that are in DNM list, there is a search box on the top, kindly search for the email address. Once you see the contact result, click ”Delete” from the Actions on the right-hand side.