Is required by law that the user postmaster is up and running
No, it is not. It is the webserver/website owners choice.
Are incorrect and unfair negative, that you blocked the sites without first contacting the postmaster.
Not a good idea. If the site is infected and/or is trying to spread malware, it should be blocked immediately and not allowing it while waiting for a reply from the owner.

If you believe your website is blocked while it shouldn’t, just report it : http://www.avast.com/contact-form.php
The people from avast will look at it and when/if needed take action to solve it.