Client adds a domain with the same name as an existing nameserver

Today I had a problem with a domain being unreachable through email. After some investigation, I found out a client added the following domain to his account: ns12.wixdns.net (likely not on purpose), which happens to be the nameserver of many wix hosted websites. So sending emails to those domains failed for every server on our network. Did I just stumble on a vulnerability? I was wondering what could be done to avoid this happening again.