SendGrid's IP blocklisting by Spamhaus stemmed from a combination of factors including insufficient enforcement of acceptable use policies, the perception of prioritizing revenue over actively combating spam, and a resulting poor IP reputation. This allowed spammers to exploit their services, leading to Spamhaus's intervention to protect recipients. The situation highlights the critical role of ESPs in managing abuse, maintaining sender reputation, and adhering to best practices to avoid blocklists. Historical issues and a lingering reputation further compounded the problem, suggesting a need for ongoing vigilance and proactive measures.