Sender Reputation: Definition, Signals, and Why It Matters

Sender reputation is a provider-specific assessment associated with a sending domain or IP address and used alongside message, authentication, and recipient signals in delivery decisions. Google exposes Bad-to-High domain/IP categories to eligible verified operators in Postmaster Tools and separately tells senders to keep the Gmail spam rate below 0.30% and aim below 0.10%. Those categories and thresholds describe Gmail, not one portable public score shared by every mailbox provider.

Verification note: This is a documentary source review checked against current sources on August 3, 2026. We did not hands-on test the named product or workflow during this review, so claims are limited to the cited documentation. Interfaces can vary by account, region, rollout, and app version.

One sentence, for the record:

Sender reputation is a provider-specific record associated with a sending domain or IP address, informed by signals that provider documents or observes, and used as one input to delivery and filtering decisions.

What is sender reputation, exactly?

Mailbox providers maintain their own sending histories and do not publish a shared formula. Google's Postmaster Tools sorts its view into four tiers — High, Medium, Low, and Bad — when enough eligible traffic exists. Those categories are descriptive Gmail telemetry, not a cross-provider verdict.

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 has reported mail sent to trap accounts that “don't solicit any mail.” A hit indicates acquisition, segmentation, or list-hygiene risk; recycled traps can include addresses that were once used by a real recipient, so it does not prove the sender never had initial permission.
• Authentication. Since February 1, 2024, senders of more than 5,000 daily messages to personal Gmail accounts must pass SPF, DKIM, and DMARC.
• Required unsubscribe controls. Gmail's bulk-sender rules require one-click unsubscribe for covered marketing/subscribed mail via the RFC 8058 List-Unsubscribe-Post mechanism. The requirement helps recipients exit; the cited provider documentation does not define “unsubscribe behavior” itself as a direct reputation-score input.
• 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	Google	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 one of the machine signals deciding what you see. When wanted mail lands in spam, the provider's internal record of the sender may have influenced the verdict alongside message and account-level signals. 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: you cannot see a sender's private Postmaster grade, but you can inspect whether a message advertises an unsubscribe mechanism and, after making a controlled request, watch whether the same list keeps mailing. Those are different observations. Flick parses List-Unsubscribe headers, but its Exit Gap Index currently publishes only a 150-sender work-list and four capability observations — not A–F honor grades. Establishing honor under the planned protocol requires at least three post-request probes over at least 48 hours with preserved evidence; the founder burner-mailbox observations have not occurred. A header therefore says nothing by itself about the sender's reputation or follow-through.

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.

Use the provider's own diagnostic before choosing a fix:

Observed signal	What it establishes	First remediation to test	What not to conclude
Gmail Postmaster spam rate rising	More measured inbox deliveries were reported as spam under Google's denominator	Pause the affected campaign, verify consent and segmentation, suppress complainers, and make the opt-out obvious	It does not identify one creative, list source, or root cause by itself
Gmail domain or IP category falls	Google's eligible telemetry worsened for that domain or IP	Compare the timing with authentication, volume, complaint, and campaign changes; restore one variable at a time	The category is not portable to Microsoft or every recipient
SNDS verdict moves toward red	A larger share of observed mail from the IP was filtered as junk in Microsoft's view	Inspect the affected IP, recent traffic mix, authentication, acquisition sources, and list hygiene	It does not prove the domain alone caused the verdict
Authentication fails	SPF, DKIM, or DMARC did not pass as expected for the sampled mail	Correct alignment and signing, then re-test with a controlled message before restoring volume	Passing authentication does not guarantee inbox placement
Recipient reports mail after an opt-out	The sender may have a suppression, identity, or list-scope problem	Preserve request/list evidence, stop the affected stream, audit suppression across aliases, and verify later mail	One message does not establish a provider-wide reputation score

For a stepwise deliverability diagnosis, use why emails go to spam; for recipient-side control, use how to stop spam emails. Change one variable at a time so a later improvement has an interpretable cause.

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." That vendor's scale runs 0 to 100. Gmail separately reports Bad, Low, Medium, or High categories in Postmaster Tools; those categories are not an equivalent or convertible scale.

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 the categories can differ. On shared infrastructure, other traffic can influence an IP-level assessment, while domain-level signals remain separate.

Does honoring unsubscribes improve sender reputation?

Honoring an unsubscribe removes one reason a recipient might later report the mail, while complaints are a documented provider signal. Gmail requires its spam rate to stay below 0.30% and requires one-click unsubscribe for covered bulk marketing mail. The sources do not quantify how much one honored request changes reputation, so treat easier exits as complaint-risk reduction, not a guaranteed score improvement.

Can a recipient see a sender's reputation?

Not directly — Gmail shows Postmaster Tools data only to the domain's operators. A recipient can inspect authentication and unsubscribe capability, then collect their own post-request evidence about later mail. Flick does not currently turn that evidence into a public sender grade: /graded has zero honor grades, so it must not be read as a reputation or compliance lookup.