How do mailbox providers score IPs?

Every major mailbox provider has a system for evaluating how much to trust an IP address before it decides where to route incoming messages. These scoring systems aren't published, but the signals they weigh are well-understood from years of deliverability research and provider documentation. Your IP's score isn't a fixed number you can look up once and forget. It's a dynamic assessment that shifts as your sending behavior changes, and recent behavior counts for more than old history.

The signals that build a strong score are the ones that look like a sender with a genuine, engaged audience: low spam complaint rates (Gmail's threshold is 0.1%, and most providers get concerned above 0.3%), consistent sending volume without unexplained spikes, low bounce rates, and clicks from recipients who actually want your messages. The signals that damage a score are the inverse: elevated complaints, sudden volume spikes after long quiet periods, high unknown-user bounces (which signal you're mailing stale or unverified addresses), and spamtrap hits. Spamtraps are addresses that should never receive mail, either because they never belonged to a real person or because they were abandoned years ago. Hitting them tells providers your list hygiene is poor.

Authentication is factored in too. Providers treat IPs that consistently send properly authenticated mail differently from those that don't. Valid DKIM signatures, correct SPF alignment, and an active DMARC policy all contribute to the trust picture. An IP with strong authentication and good engagement signals scores better than one with the same engagement signals but sloppy authentication. Google Postmaster Tools is one of the few places where you can actually see how you're scoring, at least for Gmail. It shows IP reputation and domain reputation separately on a scale from Bad to High, which helps you isolate whether a deliverability problem is coming from the IP level or the domain level.

The time dimension matters more than most senders expect. Bad behavior doesn't clear overnight. A damaged IP reputation can take weeks to recover, and the path back isn't just waiting. Providers weight recent activity more heavily than older history, so the recovery strategy is to send consistently to your most engaged subscribers first, keep complaint rates low, and gradually expand volume as the score recovers. Sending nothing while you wait doesn't help because it leaves the score stagnant without the positive signals needed to push it upward.

To see the full picture of how IP scoring fits into your deliverability setup, IP warmup explains the right sending trajectory for a new or recovering IP. And because providers evaluate IP reputation and domain reputation together, domain reputation basics complete the picture. The fastest diagnostic if you're seeing delivery problems: check your IPs against a blocklist monitor and pull up Postmaster Tools data for the past 30 days.