I do the same whenever a site requests personal info from me and I don't want to give it.

Find the corresponding info from the company or their CEO, use that.

And whenever some site wants me to register for a newsletter, I enter their own "critical support" mail address.

I hope at some point these sites learn that mandatory anything just hives you useless data.

Haha, I usually put root@localhost, or www-data@localhost. It accepts it fairly often too. Sometimes I need to open up the Chrome Dev Tools and delete the input's type attribute (<input type="email"> to just <input>) because there's some client-side thing that tries to stop you inputting invalid emails.