You don't need many unhappy people to get your server IP blocked and in fact, an incorrect DNS structure/entry and reverse IP lookup can get your domain blocked without doing a thing. What happens is that there are stringent rules to which some ISP's work with which and to be able to send alot of mail from a server, the ISP needs to ensure the server they are sending mail from has the correct Reverse DNS entry set, which sometimes goes right back to the company supplying the IP address. It's not just a matter for you to try and manage, the IP holder also at times has to make entries.
While I believe this is sometimes over the top, it's done so, simply because there is such a problem with spam. We've got about 700 mail boxes on our servers, and I have on occasions, had clients ring to tell me our servers are bouncing email. It's not forever, it's a nuisance I know, but should we drop the amount of lists we're on, we get flooded with so much spam.
We block alot of sites, but it changes, as our configuration, like many others, links to black lists. You can in fact check if your isp is blacklisted, by putting in the IP addresses and running checks on certain websites.
You can check to see if your server is configured to allow relay - if there's something not quite right, it will get on the black lists.
If your mail server has incorrect methods of how it sends it's headers, you can get rejected, not by blacklists, but by vigilant IT departments sick of handling spam.
You can problems sending mail by not being on a corporate address book - not black listed, but handled by the ISP
These are just a few things. And while I commend you on sucessfully sending out 12,000 email from your dnn site, I find it doesn't really handle unsubscribes in the correct way with many of the spam laws brought in a while ago (which I think are a bit useless myself, since most of the spam generate globally when I last read about it was in fact the US - since it's so profitable to spam for some companies when you're talking volume of mail and the minority of clickthroughs)
The more we use email, the more we are dependent on it, and the more problems we have in the delivery of it too, since it's such a misused commodity.
It might be worth your while to find a good white listed ISP and negotiate with them to see how you go with being keep in the white, instead of the black!
I sympathise with your situation, but having been on the receiving end of so much spam, I just accept that as part of mail delivery, some won't reach it's destination. I don't like it, but that's just some of my experiences with mail over the last 6 years.
Nina Meiers