Of course, you should make every possible effort to have a real name on the letter. State Enforcement Guy has stooped to phoning the company and confessing he had a letter to send to the (insert position title here) and didn't know the name. If there's a live receptionist, they're usually quite cheerful about giving you the right name. (Given that letters for me are usually trouble, this might be evidence of a little hostility within the company's ranks. But it works anyway.)

If you call and the receptionist refuses to give you the name, it's likely that anonymity is their policy. Perhaps for security reasons; it's hard to be too careful. If that's the case then they blamed well ought to be expecting a nonspecific greeting, so it won't ding you any in their estimation.

We use "Dear Sir or Madam," and since we're a boring state agency this is probably the most boring, expected, conventional option available to you. Which is not a bad thing. In my personal correspondence I have used Dear Friend: but that's a bit presumptious even in an era of "Friends Lists" that may or may not have anything to do with genuine friendship.

Best of luck.