When a crazy person from your past finds you through Google, what do you do?
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As far as I can tell, this is a multiple choice question.
(a) Pretend you never got the email.
(b) Respond, and try to somehow politely say that you don't think you and said person have much in common anymore (and somehow try to refrain from pointing out that the content of the out-of-the-blue email only confirms said person's lunacy).
(c) Send some snarky response in reply.
(For purely hypothetical purposes, let's say that this is a crazy ex-roommate, not an ex- of another kind -- so it's both more and less complicated than it otherwise would be.)