How do ESPs decide which IPs to use for your account?
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So when When you sign up with an ESP, you don't get to pick your IP. The ESP makes that call based on a few factors, and understanding what they're looking at helps you understand why your deliverability starts where it does.
New accounts almost always start on shared pools. The ESP has no track record for your sending, so they put you somewhere with established reputation while you build your own. As your volume grows and your metrics look good (low complaints, low bounces, decent engagement), you may get moved to better pools or offered a dedicated IP option.
Most ESPs segment their shared pools by sender quality. Good senders end up together. Problem accounts get isolated. Your complaint rates, hard bounce rates, and engagement signals all feed into where you sit in that hierarchy. If your metrics slip, you might find yourself in a lower-quality pool without being told explicitly.
Enterprise accounts often receive dedicated IP infrastructure by default, because at that volume the ESP doesn't want your sending mixed with self-serve customers anyway. Self-serve accounts typically share infrastructure unless they purchase dedicated IPs as an add-on.
Your sending reputation influences pool placement, and pool placement influences reputation. Keeping your metrics clean, especially complaint rate, is the most direct way to influence which neighborhood your sending lives in.
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