When running a private network with DNS the network admin will set up .localdomain or .local as TLD for the private network.

But that's so many letters! Single letter TLDs are not assigned to anyone. While I'm pretty sure that's not an official rule, it would seem that they are specifically excluded. See the current list.

Bind 9 and Route53 both accept single letter TLD's. Is your company named Example? Set your private TLD to .e.