How do ESPs pool IPs?

ESPs use sophisticated strategies to group senders into IP pools. The goal is protecting good senders while isolating risky ones.

Quality-based pooling: Senders are scored on complaint rates, bounce rates, and engagement. High-quality senders share pools with other good senders; problematic senders are isolated or moved to lower-tier pools.

Volume-based pooling: High-volume senders may get dedicated pools or IPs. Low-volume senders are grouped together since they can't sustain individual IP reputation.

Industry segmentation: Some ESPs pool senders by industry. Financial services, e-commerce, and media might have separate pools since their sending patterns and recipient expectations differ.

Risk tiering: New accounts often start in probationary pools until they prove sending quality. As metrics improve, they're promoted to better pools.

Dynamic assignment: Advanced ESPs move senders between pools automatically based on real-time performance. A sender whose metrics degrade gets shifted to a lower pool before they damage premium infrastructure.

You usually can't choose your pool on shared infrastructure. Your performance determines your placement. This is why following best practices matters even on shared IPs: better behavior earns access to better pools.