How do ESPs pool IPs?
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If you send through a shared ESP, you're not sending from a single IP address that belongs only to you. You're sharing infrastructure with other senders, and the ESP groups those senders into pools. Which pool you land in depends on how you behave.
Quality-based pooling is the most common approach. ESPs score senders on complaint rates, bounce rates, and engagement. Senders with good metrics share pools with other well-behaved senders. Senders with high complaint rates or dirty lists get moved to lower-tier pools, or isolated so their problems don't drag down the rest.
Volume-based pooling is also standard. High-volume senders sometimes get dedicated IPs or pools because the volume alone lets them build their own reputation. Low-volume senders are grouped together since none of them individually can send enough to establish a reputation on their own.
But Some ESPs also pool by industry. Financial services, e-commerce, and media tend to have different sending patterns and different recipient expectations. Keeping them separate avoids one industry's norms bleeding into another's reputation.
On most shared infrastructure, you can't choose your pool. Your sending behavior determines your placement. Better complaint rates, lower bounce rates, and higher engagement move you toward better pools. That's the direct incentive structure that makes list hygiene and authentication worth doing even when you're not on a dedicated IP.
If you're on a shared plan and worried about pool placement, your ESP's deliverability support team should be able to tell you what tier you're in and why. Or if you're hitting delivery problems you can't explain, the SOS hotline is free.
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