What’s the difference between IP and domain scoring?

Short version: IP scoring judges the machine that sent the message. Domain scoring judges the brand that signed for it. Mailbox providers use both, and the weights have shifted hard toward domain over the last decade.

IP reputation: the sending server's track record

Every email is delivered from an IP address. Mailbox providers like Gmail, Yahoo, and Microsoft keep a running file on that IP: how much mail it sends, how often recipients mark it spam, how often it hits spam traps, whether it shows up on public blocklists. If you send from a shared IP at an ESP like Mailchimp or Brevo, you share that file with every other customer on the same address. One bad sender on a shared pool can drag your delivery down even if your own list is clean. That is why dedicated IPs exist, and why warming a new IP from zero to production volume takes weeks, not days. For more on how providers use this data alongside other signals, see how spam filters work.

IP reputation is tied to one number. The minute you change ESPs, you start over.

Domain reputation: the brand's track record

Domain reputation follows your sending domain across whichever IP you happen to use. If you move from SendGrid to Postmark tomorrow, your IP reputation resets but your domain reputation comes with you. That is the part most senders underestimate. Years of complaints, unsubscribe rates, and spam trap hits stick to the domain in the From header and the domain in your DKIM signature.

The mechanic that makes this work is authentication. DKIM signs the message with a cryptographic key tied to a specific domain, declared in the d= tag of the DKIM-Signature header (RFC 6376). When Gmail or Microsoft sees a valid DKIM signature with d=yourbrand.com, they have proof that yourbrand.com authorised this message. They can safely attach reputation to it.

DMARC layers on top. It only passes when the domain in the visible From header aligns with either the SPF return-path or the DKIM d= domain (RFC 7489). That alignment check is what makes domain reputation trustworthy. Without it, anyone could forge your From address and poison your score. With it, the score belongs to you.

Google has been explicit about this. Postmaster Tools reports both an IP reputation dashboard and a domain reputation dashboard, and Google's own guidance is that domain reputation now carries more weight for established senders.

How the two combine in practice

A modern filter does not pick one or the other. It looks at both and reacts to the weaker signal.

• Good IP, bad domain: you still land in spam. The domain history outweighs the clean infrastructure.
• Bad IP, good domain: you might survive at Gmail, less so at Microsoft, which leans harder on IP signals.
• Both good: inbox.
• Both bad: junk folder, or outright rejection at the SMTP layer. See 5xx permanent vs 4xx temporary rejections for what the bounce codes actually mean.

The practical takeaway. If you only watch your IP, you are watching the cheap, replaceable part. Domain reputation is what you build over years and what you cannot escape by switching ESPs. Authenticate properly with SPF, DKIM, and DMARC. Keep your list clean. Watch both dashboards in Postmaster Tools, not just the one that looks better this week.