Spamhaus or another IP reputation provider will contact your hosting provide or ISP and warn them that either: - You need to follow their best practices (which practically for me meant paying for a subscription) - Or your upstream net block would be marked as untrustworthy (which basically blocks email delivery from that IP range)

You can imagine what your hosting provider or ISP will do with this.

Source: I ran a URL shortening service from 2004-2007 and this happened to me.