I'm not sure where you'd run this. Most cloud compute providers intentionally list their network blocks on blacklists to reduce their attractiveness to spammer

I'm not sure where you'd run this. Most cloud compute providers intentionally list their network blocks on blacklists to reduce their attractiveness to spammers. Some consumer-class broadband providers do the same and some even block inbound port 25. There's also the issue of reputation-based classification of incoming email at the major providers--you may not do well against those measures using this.