Email domain no longer exists?

I received an email to my personal email from one of my customers with the below info:  What is going on with the email service and how will this be fixed??

Delivery has failed to these recipients or groups:

@ourdomain
Your message couldn't be delivered. The Domain Name System (DNS) reported that the recipient's domain does not exist.

Contact the recipient by some other means (by phone, for example) and ask them to tell their email admin that it appears that their domain isn't properly registered at their domain registrar. Give them the error details shown below. It's likely that the recipient's email admin is the only one who can fix this problem.

For more information and tips to fix this issue see this article: https://go.microsoft.com/fwlink/?LinkId=389361.