Is it on spec? Somehow I doubt there's an actual teapot involved anywhere in responding to that request.
[1]http://en.wikipedia.org/wiki/Hyper_Text_Coffee_Pot_Control_P...
The idea was that I might have domains configured on my DNS which resolved to the same machine. And if you connect to that machine on port 80, no matter what domain, you will get the HTTP server.
The problem is, the DNS might recognize domains which the HTTP server is not listening for, e.g. "mail.drostie.org" or some such. When you visit those in a web browser, the server can't do what it's supposed to do -- pretend it's not listening on port 80 -- because it already received the HELO and the GET.
It's also clearly a client error, so the error code should be in the 4xx range. But just go down the list: it's not a syntax error (400), it wasn't unauthorized (401), payment isn't required (402), and the server technically didn't even understand the request, so it's not 'I understand but I refuse to fulfill' (which is 403). It's not that some file was not found (404) or that GET is not allowed (405) or that we couldn't fit the Accept HTTP header (406).
The real problem is that you asked mail.drostie.org to do something other than handle mail. None of the 4xx status codes really fits, and the one that comes closest is... 418. You asked me to brew coffee, but I'm a teapot.