A. Kudos for having a custom HOSTS file installed, Don. It's a great tool for making your browsing experience safer. But it does require occasional tweaking, as you've discovered.
However, I am curious about the example you mentioned. Atdmt.com is a part of Microsoft's ad network. It handles advertising and customer tracking. Blocking it shouldn't affect a Website's core functionality. Still, it won't hurt to do a little HOSTS editing.
I haven't talked about the HOSTS file in a while. So, now is a good time for a quick refresher course on the HOSTS file. It isn't too complicated, but it can get confusing.
As you may know, every computer has a unique IP address. That goes for Websites as well. My Website's address, for example, is 188.8.131.52. IP addresses are critical for the Internet to work. They help computers and gadgets find your computer, gadget or Website. That's how information gets to the right place.