How can one list affect multiple ESPs?

When an IP address is blocklisted, all senders using that IP are affected. On shared IP infrastructure common with many ESPs, this means one sender's bad behavior harms everyone sharing that resource.

ESPs mitigate this through monitoring and enforcement. They watch their IP pools for reputation problems and act quickly when issues arise. Senders violating policies may be suspended to protect the shared resource.

However, damage can occur before enforcement catches the problem. A sender who suddenly sends spam can trigger a blocklisting that affects other customers for hours or days.

This shared fate is why many ESPs offer dedicated IPs for larger senders. With dedicated infrastructure, your reputation is your own, isolated from others' behavior.

Shared vessels share their fate. The actions of one crew member affect everyone aboard.