This domain is only here to let people put email addresses of valid form in their GitHub commits without fear of spammers harvesting email addresses. It is in no way affiliated with GitHub.com
Set the email address in your git config to something @hubuser.com. For the sake of uniqueness I'd recommend using your GitHub username. So mine would be nuclearsandwich@hubuser.com
git config --global user.email nuclearsandwich@hubuser.com
Now add this email address to the list of emails attached to your account on GitHub.com and just don't verify it.
You can't use this email for GitHub Pages since pages only build for verified email addresses.
Again, this has no affiliation with GitHub and is just here to save people who are paranoid about spam. I doubt I will ever set up an MX record for this domain but if I do it will be because Big Brother told me to and all it will have is an admin@hubuser.com that forwards to my proper email address.
Love and kittens, Steven! aka @nuclearsandwich