What is IP pooling?
Still have a question, spotted an error, or have a better explanation or a source we should cite?
When you send through a shared ESP, your messages don't usually go out from a unique IP address that belongs to your account. They go out from one of a group of IPs that the ESP has assigned to senders with similar profiles, which is IP pooling. The pool handles the traffic load, and mailbox providers score the pool's sending history collectively. That means your deliverability is partially tied to the behavior of everyone else sending through those same IPs.
That shared scoring is the thing worth understanding. Imagine you're sending clean, well-engaged campaigns to a healthy list. Another sender on the same IP pool buys a list and ignores complaints. Their complaint rate climbs, which damages the pool's reputation score, which can drag your inbox placement down too. You didn't do anything wrong, but you're sharing the consequences. This isn't the most common scenario, but it happens enough that high-volume senders actively want to know which pool they're on and how it's performing.
Most ESPs manage pool assignment automatically. New accounts start on entry-level shared pools and migrate to better-performing pools as their sending history builds up. Some ESPs segment pools by industry, volume tier, or engagement level. Klaviyo and SendGrid both offer dedicated IP options at higher tiers for senders who want to own their IP reputation entirely. With a dedicated IP, you're the only one building (or damaging) its standing. That's a benefit if your sending is consistent and your list is healthy, and a liability if you send infrequently or your list has gaps, because an underused IP doesn't build the reputation it needs to perform well.
If you're seeing unexplained deliverability dips despite good list hygiene and engagement, the pool is worth investigating. Check whether your sending IPs have picked up blocklist listings (a quick lookup through MXToolbox or similar tools takes about 30 seconds), and ask your ESP which pool tier your account is on. IP reputation monitoring covers the tools and thresholds in more detail. And if you're evaluating whether a dedicated IP makes sense for your volume, dedicated vs. shared IP tradeoffs walks through the decision. The general threshold: you typically need to send at least 50,000 to 100,000 messages per month to keep a dedicated IP warm enough to build consistent reputation.
Contributors
Who worked on this answer
Every name links to their profile. Every company links to their site. Real people, real accountability.
