Ask HN: Am I spamming my customers?

Hi, I'm looking for advice as to whether it's legitimate for me to send a product upgrade email to a group of people who have a license for my product. Here's the story: I'm an independent developer and licenses for one of my products was given away in a promotion run by a third party earlier in the year. A condition of getting the license was that they had to register with the third parties site and then go through a virtual purchase process which cost them nothing. At this point I don't think there was any kind of check if they were willing to receive email. At the end of the promotion the third party supplied me with the email addresses of the people who downloaded the license, about 30,000 of them. The product has changed significantly since then and I 'd like to send these people an email informing them about the changes but I'm concerned at being branded a spammer and the potential backlash against the product. What do you think, can I email these people even though they haven't explicitly signed up for it? My gut feeling is that I shouldn't but I'm wondering if I'm being over cautious here?