Spam or legitimate marketing?

I have just had such a mixed response from contacting one sector of industry with emails advertising my product. A once off very brief ad, with a summation of the benefits of my product and with my details very clearly stated, etc., asking people to make contact if they were interested in knowing more. Admittedly I contacted this sector through a resource channel that they all use. A few very positive feedback emails, and then a few really irate ones!

I was shocked at the agression of the bad responses, (actually quite distressed) as I had truly acted in innocence and not thinking I could possibly offend.

This is a definition I got off the web re spam vs. legitimate marketing:

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Define spam.
What is it and how does it differ from legitimate marketing efforts via e-mail and the Internet? Is there a difference? One person's spam is another's marketing e-mail shot.

Michael Brunzlik, MD at TSI: Spam is repetitive unsolicited e-mail. It differs from a single unsolicited e-mail from a credible marketer...
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Surely in this day and age online/email marketing is a legitimate tool?

Your opinions would be valued?