loosecannon,
While I understand tht you may have had good intentions by sending a mass mail, the fact that you inform people that they are on a mailing list and that they need to unsubscribe in order to remove themselves from it is unforgivable. If you are in the network security game, you should know full well that the worst spammers use so called "unsubscribe" links to intentionally validate active email addresses, which they can then sell for a higher price.
If you felt that you had to take action, a courtesy mail would have ben sufficient. Something along the lines of "I recently received a user list from Sentech, thought you should know. After sending this mail, I will delete it as well as all your private information that is stored on my system."
In fact I urge you to do just that, delete the information immediately. You do realise that if any spammers etc. do get hold of this info, that you will be suspect number 1. Not good for your rep in the IT industry.
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