IIS with an exchange back-end isn't responding to valid queries, but will throw 400 Bad Request for invalid queries.
From localhost PS:
> telnet 127.0.0.1 80 GET / HTTP/1.1 HTTP/1.1 400 Bad Request [ etc. ] > telnet 127.0.0.1 80 GET / HTTP/1.1 [no reply]
This is the same error from other hosts, and IIS is listening on 443, 80, and 444.