Sender Reputation: Definition, Signals, and Why It Matters
Sender reputation is the trust score mailbox providers like Gmail and Outlook assign to a sending domain or IP address, based on how recipients react to its mail. It decides whether a message lands in the inbox, the spam folder, or nowhere at all. The thresholds are public: Gmail tells senders to keep spam complaint rates below 0.10% and never above 0.30%, and grades every sending domain from Bad to High in Postmaster Tools. Reputation is earned slowly and lost quickly.
One sentence, for the record:
Sender reputation is the running record a mailbox provider keeps on a sending domain or IP address — built from spam complaints, spam-trap hits, authentication, and unsubscribe behavior — that determines whether its mail reaches the inbox.
What is sender reputation, exactly?
Every mailbox provider keeps a memory of your sending history. Google's Postmaster Tools sorts that memory into four tiers — High, Medium, Low, and Bad. "High" means a "history of very low spam rates" with mail "rarely marked as spam." "Bad" means mail is "almost always marked as spam or rejected by the receiving server." There is no appeals desk.
It is really two ledgers. Domain reputation follows your sending domain wherever it goes. IP reputation attaches to the server address the mail leaves from — shared with strangers or owned outright. Providers track both, so a clean domain on a burned IP still has a problem, and vice versa.
What signals feed email sender reputation?
The inputs providers document, in rough order of pain:
- Spam complaints. Gmail's sender guidelines say to keep the rate reported in Postmaster Tools below 0.10% and "avoid ever reaching a spam rate of 0.30% or higher." That is recipients hitting "report spam."
- Spam-trap hits. Microsoft's SNDS counts mail sent to trap accounts that "don't solicit any mail." Hitting one proves you never had permission.
- Authentication. Since February 1, 2024, senders of 5,000+ daily messages to Gmail must pass SPF, DKIM, and DMARC.
- Unsubscribe friction. The same Gmail rules require one-click unsubscribe via the RFC 8058
List-Unsubscribe-Postheader — see our one-click unsubscribe explainer. - Filtering history. SNDS colors each IP by verdict rate: green under 10% junked, red above 90%, yellow in between.
Where can you check sender reputation?
Three instruments cover the major providers:
| Tool | Run by | What it scores | Scale |
|---|---|---|---|
| Postmaster Tools | Domain and IP reputation at Gmail, plus spam rate | Bad / Low / Medium / High | |
| SNDS | Microsoft | Per-IP filter verdicts and complaint rate at Outlook.com | Green / yellow / red |
| Sender Score | Validity | IP reputation from data across 80+ mailbox and security providers | 0–100 |
Two footnotes. Sender Score calls 80 and above good, and below 70 in need of repair. And Microsoft has said trap-hit counts leave the SNDS data report on July 22, 2026 — one classic signal going dark for the senders watching it.
Why does sender reputation matter if you're a recipient?
Because it is the machine deciding what you see. When wanted mail lands in spam, a reputation lookup put it there — the sender's record, not your settings. We cover the mechanics in why emails go to spam. If your problem is the opposite — mail that keeps arriving — start with how to stop spam emails.
Here is the part most definitions skip: one reputation signal is observable from the outside. You cannot see a sender's Postmaster grade, but you can see whether they honor an unsubscribe. We parse List-Unsubscribe headers for a living at Flick (flicked.email), and we publish what we find: the Exit Gap Index grades real senders A–F on whether unsubscribes actually stick. It is our own instrument, built from our own crawler — not a neutral third party — so read it as such. But a sender who keeps mailing after you opt out is telling you, and every spam filter watching, exactly who they are.
How do senders protect their reputation?
Slowly and boringly. Send to people who asked. Authenticate everything. Keep complaints under Gmail's 0.10% target. Make leaving effortless — a recipient who cannot leave reaches for "report spam," and that counts against the ceiling. If mail is already landing in spam, the troubleshooting checklist lives in why emails go to spam.
Stop reading your inbox. Start flicking it.
Flick turns every inbox into a finite swipe deck — archive, "no reply needed," or AI-draft → approve, one card at a time. Inbox flicked.
Try the live demo — no signup →FAQ
What is a good sender score?
A Sender Score of 80 or above is considered good; Validity says scores below 70 signal an IP "flagged for engaging in risky sending behaviors." The score runs 0 to 100, like a credit score for a sending IP. Gmail's equivalent is a "High" domain rating in Postmaster Tools.
What is the difference between domain reputation and IP reputation?
Domain reputation follows the sending domain; IP reputation follows the server address the mail leaves from. Google tracks both separately in Postmaster Tools, so a sender can hold a clean domain record and a poor IP record at once. Senders on shared IPs inherit their neighbors' behavior, for better or worse.
Does honoring unsubscribes improve sender reputation?
An honored unsubscribe removes a future complaint, and complaints are the metric with a published ceiling. Gmail requires spam rates to stay below 0.30% and requires one-click unsubscribe for bulk senders. Every opt-out a sender ignores is a "report spam" click waiting to happen — which is why unsubscribe-honoring behavior is worth measuring in the open.
Can a recipient see a sender's reputation?
Not directly — Gmail shows Postmaster Tools data only to the domain's operators. What a recipient can observe is behavior: whether mail authenticates, whether an unsubscribe link exists, and whether opting out actually stops the mail. That last one is the signal we grade publicly, sender by sender.