Why did domain reputation replace IP reputation as the main signal?

IP reputation used to be the main signal because, in the early 2000s, that was the only stable thing to score. One IP, one sender, one mail server sitting in a rack somewhere. Score the IP, and you scored the sender. That model broke.

Here is what changed. Senders stopped owning their own infrastructure. They moved to shared ESPs like Mailchimp, Klaviyo, SendGrid, Postmark, where thousands of brands share the same pool of IPs. They moved to cloud platforms (AWS SES, Google Cloud, Azure) where IPs rotate and get recycled across tenants. They started sending from IPv6 ranges, where there are so many addresses that tracking individual ones is meaningless. A /64 IPv6 block alone holds 18 quintillion addresses, which makes per-IP reputation a non-starter (M3AAWG IPv6 sender guidance).

So the IP stopped identifying who was sending. The domain in the From header, and the domain in the DKIM signature, became the only things that stayed constant when a sender changed ESPs or scaled to new infrastructure.

The bigger problem with IP reputation was abuse. A spammer could burn an IP, abandon it, and grab a fresh one within hours. There was no permanent cost. By tying reputation to a domain (which costs money, requires registration, and shows up in WHOIS), mailbox providers forced sender identity to carry weight. You cannot register reviewmyemails.com, burn it, and grab a new one without losing the brand. That asymmetry is the whole point.

Gmail made this shift official in 2016 when Postmaster Tools launched with domain reputation as a primary metric (Google Postmaster Tools FAQ). Microsoft, Yahoo, and the rest followed. Today every major filter treats the From domain (and the DKIM d= domain) as the primary identity signal, with IP as a secondary input. For the details on how those two interact, see our breakdown of IP reputation vs domain reputation.

For you, the legitimate sender, this is good news. Your domain reputation follows you. If you spent two years building trust on Mailchimp and you migrate to Klaviyo, the new IPs are unknown to Gmail, but your sending domain is not. The reputation you built does not reset.

A few practical implications:

• Pick your sending domain carefully. Once it has reputation, you do not want to throw it away. See how long it takes to build email reputation for the timeline.
• Subdomains inherit (partially) from the root. A bad cold-email subdomain can drag the parent down. Read can one bad subdomain harm your main domain before you spin up marketing.brand.com for risky sends.
• DKIM is what carries the signal. The DKIM d= value is the domain that gets scored, which is why DMARC alignment matters. Details in how DMARC strengthens domain reputation.
• IP still matters, just less. A brand-new dedicated IP with no history will hurt you during warmup even if your domain is healthy. The two scores combine; they do not replace each other entirely.

The short version: domains became the primary signal because they are the only thing senders cannot cheaply abandon, and the only thing that stays the same when everything else (ESP, IP, infrastructure) changes underneath.