Well, I'm using one of those www pages that checks in both directions for you. The line "could be forged" definitely makes me think of e-mail rather than www site visitors-- and I don't think I have ever in my life received an e-mail whose content left me in doubt about whether it was spam or real mail. (I'm an individual human, not an ISP, so I don't have to deal with the messy issue of decoding forged headers and blocking senders at the source.)
So far I haven't found anything that works better than gut feeling: page A gets a fair number of visitors from Portugal, so I don't bother about them, but why the ### should people from Belarus suddenly take an interest in page B?
Most of the time, "does not exist" seems to refer only to the exact wording of the address. The domain part is the same. Or it isn't, at which point it is probably easier to deploy the "because I don't like your face" rule than to bother with any more investigation. The address that prompted the original question is, as I said, administered by people who are moderately clueless* so I wouldn't put it past them to assign a local address in a non-standard format.
* They're actively promoting a related site that has, to my certain knowledge, not been touched since 2004, which gives you some idea.