I don't trust email to be secure enough for this; they may still be going out in plaintext. How do you automate DNS nameserver updates if you are migrating hundreds of domains?
In the case of a large-scale, multi-domain authoritative nameservers assignment change, only the DNS provider would be able to facilitate the process programmatically. And at that kind of scale (i.e. hundreds of domains), one would hope verification would not only be obtained through, but require some form of human-to-human contact coupled with thoroughly documented scope.