Question: If I just want to know if a domain name is reserved; is it sufficient to use this command and see if any domain name servers turn up, in which case it's reserved?

```
host -t NS example.com

```

It's a lot faster than visiting <http://www.internic.net/whois.html> and typing example.com to get much more detailed results, which I'm not interested in anyway.

Answer: 
Absolutely not.

A past employer registered theirname.biz solely for use on the internal network: it had DNS entries on the inward-facing network DNS server, but nowhere on the internet.

I'm not sure the trick was particularly essential, but "imap.theirname.biz" has the advantage over just "imap" that it's unambiguous if you're connected simultaneously to multiple networks (in the absence of deliberate foul play, of course), so you can just use all their internal DNS resolvers. Also the advantage over "imap.theirname.com" that once you know the convention, it's immediately obvious that it's a private server, and hence the reason you can't connect to it is that you forgot to connect VPN. There may have been other benefits to which I was not privy: I'm a coder, not an IT tech...
I don't believe that you have to have a DNS pointing to your domain. Even if you had to have a DNS set up, there is no assurance that the box acting as the DNS server isn't down.