Wesley S, I accept your basic premise, but I wish we would drop the overly-inclusive term "negative campaign." Unfortunately, it's become a broad-brush term used to refer both to real "mudslinging" (including blatant lies about people's private lives and character) and to fact-based reports or allegations about candidates' voting records, policies, etc.
I find absolutely nothing wrong with pointing out wherein candidates' voting records and policy positions differ, including drawing attention to really bad consequences (past, present or future) of certain policies and legislative or executive actions. In fact, I think this sort of report (or ad or email), if accurate, can be of much more value to the voters than just "positive" promises. (An ideal campaign presents the positives of one's own policies and record and contrasts those with the negatives of one's opponents'.)
So, if an email points out cases where Hillary, e.g., supported or acceded to some of Dubya's worst nominations or policies, I think that's not only fair but very valuable. If it's based on libel or even just sleazy innuendo, that's a different thing, and I agree that that's where people turn when they have no legitimate basis for opposing a candidate.
As for unsolicited emails, I hardly ever open them, regardless.