Sender reputation is the overall judgment a mailbox provider forms about you as a sender, pieced together from the reputation of your sending domain and IP address, whether your mail is properly authenticated, and how recipients treat what you send. It decides whether your next message reaches the inbox, lands in spam or gets refused before anyone sees it.

There's no single sender reputation sitting somewhere for you to look up. Gmail keeps its own view of you, Microsoft keeps another, Yahoo a third, and the security filter in front of a bank's mailboxes keeps a fourth. They read similar evidence and reach different verdicts, which is why "our sender reputation is fine" usually means one tool, about one provider, said so.

That fragmentation is where teams go wrong. Replies fall, the blended numbers show nothing obvious, and the copy gets rewritten three times while the real problem sits with one provider that has started routing everything to junk.

TL;DR

• Providers build sender reputation from four layers: your domain, the IP the mail leaves from, authentication, and what recipients do with your messages. The domain carries the most weight today, and the comparison of domain, IP and sender reputation lives on the domain reputation page.

• If you send through Google Workspace or Microsoft 365, the IP belongs to the platform and is shared with other customers. What you control is the domain, the list and the way recipients react.

• Gmail, Microsoft and Yahoo keep separate books. Gmail leans on the domain and user spam reports, Yahoo rates every IP and DKIM domain, and Microsoft's SNDS only reports on IPs.

• B2B email meets a second judge at the recipient's company. A Microsoft 365 admin can lower the bulk mail threshold from the default 7 to 5, and a secure email gateway runs its own reputation checks on top.

• Lookup scores such as Validity's Sender Score rate an IP from 0 to 100. Treat them as a hint, then split seed tests and reply rates by recipient provider to see where you actually stand.

• Repair works per provider: find which one turned against you, cut volume to it, send only to people likely to engage, and fix the cause before ramping back up.

The four layers behind a sender reputation

When a message arrives, the receiving server isn't scoring the email in isolation. It's checking what it already knows about everything attached to that message, and adding the new evidence to the pile.

The first layer is the domain, and today it's the heaviest. Domain reputation follows your domain across every tool and server you send from, so switching sequencers doesn't wipe it. That page also has the side-by-side table of domain, IP and sender reputation, which we won't repeat here.

The second layer is the IP address the mail leaves from. For most B2B teams this matters less than they think, for an unglamorous reason: if your mailboxes run on Google Workspace or Microsoft 365, the IP isn't yours. Google's sender guidelines put it plainly, saying the activity of any sender on a shared IP address affects the reputation of every sender on it. You inherit the platform's IP standing, and your own behavior shows up mostly against your domain.

The third layer is authentication. SPF, DKIM and DMARC don't make you trusted. They make you identifiable, so the provider knows which domain to credit or blame. Mail that fails them has no stable identity to build reputation on, and some providers reject it outright.

The fourth layer is behavior, and it moves fastest. Spam reports pull hardest. Google says user spam reports can lower your domain's reputation over time, and it wants the spam rate in Postmaster Tools under 0.1%, which sits inside a wider set of bulk sender rules covered under email deliverability. Bounces count too, since a high bounce rate tells the provider your list wasn't checked. On the positive side, replies and people dragging your message out of the spam folder count in your favor, and silence from the same people month after month counts against you even when nobody complains.

Notice what isn't on the list: your subject line, your offer, your brand. Content filters look at the message, but reputation is about history. A great email from a sender with a bad record still lands in spam.

Gmail, Microsoft and Yahoo each keep their own sender reputation

Every provider sees the same four layers but weights them differently and shows you different slices. Knowing the differences tells you where to look when one of them turns.

Gmail

Gmail is the most domain-centered of the three. User spam reports feed straight into how it sees your domain, and so does the opposite signal, a recipient marking your message as not spam. Its feedback loop is built for large senders and works in aggregate: you add a Feedback-ID header to your mail and Gmail reports complaint levels per identifier, not per person. Postmaster Tools is the closest thing to a window, and Google has been reshaping what it shows, which the domain reputation page covers in detail.

Outlook.com and Hotmail

Microsoft's consumer mail has the longest-running sender program. Smart Network Data Services (SNDS) gives senders detailed data about individual IPs, and the same site runs the Junk Email Reporting Program, which sends you a report when users mark your messages as junk. The catch for most outbound teams: SNDS is about IPs you register, and if you send through Microsoft 365 or Google Workspace you don't own the IPs. Your view of Microsoft is mostly what your seed tests and reply rates tell you.

Yahoo

Yahoo is explicit that it rates two things. Its sender best practices say each IP and each DKIM domain has a reputation that can affect delivery, and its Complaint Feedback Loop is tied to the domains you sign with DKIM. It asks bulk senders to stay under a 0.3% spam rate, and it calculates that rate on mail delivered to the inbox, so your own figure based on all sent mail can look better than Yahoo's. For most B2B lists Yahoo is a small share of recipients, but it's the clearest about its own rules.

Selling to businesses adds a second judge

Consumer mailboxes get one verdict. Business mailboxes often get two: the platform's, and the company's own.

Microsoft 365 is the obvious case. Microsoft stamps a bulk complaint level, from 0 to 9, on mail it identifies as coming from bulk senders, and each customer's anti-spam policy decides what happens at which level. According to Microsoft's bulk email documentation, the default threshold is 7, the Standard preset uses 6 and the Strict preset uses 5, and mail at or above the threshold goes to junk or, under Strict, to quarantine. The same sequence step can reach the inbox at one company and junk at the company next door, purely because two IT teams chose different settings.

Larger companies often put a secure email gateway in front of their mailboxes as well. Those gateways keep their own reputation data and apply their own rules, so a sender in good standing with Microsoft can still be held at the gateway. When a campaign does well with small companies and goes silent at enterprises, this second judge is usually the reason.

Scores you can look up, and what they leave out

Because providers don't publish their verdicts, a small industry of lookup scores fills the gap. They're useful as long as you know what they measure.

Validity's Sender Score, the one most people have heard of, describes itself as a credit rating for your IP address, scored from 0 to 100. It's built from data Validity collects, not from Gmail's or Microsoft's internal scores. For a team sending from Google Workspace, it mostly describes Google's shared IPs, which tells you little about your own domain.

Blocklist checks are more concrete. A listing is a yes or no, and some providers act on the major lists. Postmaster Tools covers Gmail only, and SNDS covers IPs you register with Microsoft. None of these will tell you how Microsoft 365 tenants or gateways see you.

So the reading we trust most is the one you take yourself. Seed tests split by provider give you an inbox placement rate for Gmail, Outlook and Yahoo separately. Reply rates split by sending domain and by recipient provider show which pairing is slipping. An outbound deliverability audit puts those readings together with the authentication and list checks, so you know which layer moved.

Why does a sender reputation problem look like a copy problem?

Because the numbers most teams watch are blended. Picture a list where roughly half the contacts sit on Microsoft 365 and half on Google Workspace. If Microsoft starts sending your mail to junk, the overall reply rate drops by about half, and nothing in the sequencer says why. The emails still show as sent. Nothing bounced. The natural conclusion is that the message stopped working.

Split the same numbers by recipient provider, which you can do by checking the MX record of each contact's domain, and the picture changes. Google recipients reply at the old rate, Microsoft recipients have gone silent. That's a reputation problem with one provider, and no rewrite will fix it.

The habit worth building is to tag every contact with its mail provider at the point of list building, so every report can be split that way later. It costs one lookup per domain and it's the fastest way we know to tell copy problems from reputation problems.

Running sender reputation across a fleet of domains

Cold outreach at any real volume runs on several secondary domains and many mailboxes, which turns reputation from one thing to protect into dozens. That's deliberate. Good cold email infrastructure keeps the risk of cold sending away from the company's main domain, where invoices, support replies and responses to inbound demo requests have to arrive. When a cold domain goes bad, you lose one domain, not your ability to email customers.

A fleet only helps if each domain is run with care. Every new domain goes through email warmup before it carries campaigns. Volume is spread with inbox rotation so no single mailbox carries the load, and each mailbox is kept to a modest daily pace through email throttling. Every list passes email validation before it reaches a sequence, and in our builds an inbox gets paused when its hard bounces pass about 3%, because at that point the list is the problem.

The fleet also needs bookkeeping. You want to know, per domain, which lists and which sequences it has carried, so that when one domain starts slipping you can tell whether the cause is that domain or a list that's about to hurt every domain it touches.

Repairing sender reputation one provider at a time

Recovery starts with the split described above. Find out which provider turned, and whether it's all your domains or one. A single domain slipping at one provider is a local problem. Every domain slipping at once points to a shared cause, usually a list source or a sequence that's drawing complaints.

Then reduce what the struggling provider sees from you. Cut volume to that provider, keep mailing the people who replied or engaged recently and nobody else, and pause sequences with long chains of follow-ups to people who never answered. Fix the cause before you raise volume again, whether that's a list source, a broken DKIM record on one domain, or a step that's generating spam reports. Then ramp back up slowly.

Expect this to take weeks. One clean week proves little to a provider that has months of history on you, and Google's own spam rate can lag a fix by as much as seven days. If a secondary domain is still struggling after a month of careful sending, retiring it is often cheaper than nursing it, and the domain reputation page walks through that decision.

The bottom line

Sender reputation is less a score than a set of verdicts, one per provider and sometimes one per company, each built from the same evidence: whose domain this is, where it came from, whether it's authenticated, and how people reacted last time. You can't read any of them directly, so you manage the evidence and measure the outcome provider by provider.

That shifts where the effort goes. The authentication records take an afternoon. The real work is in the list, the pace and the targeting, because those decide whether recipients reply, ignore you or hit the spam button. The top outbound experts look at reputation before they look at copy for that reason, and it sits underneath any GTM system that leans on email long before targeting or messaging come into play.

The sending setups we build live in the client's accounts from the first day: the domains, the warmup schedule, the reports split by provider and the pause rules, plus a one-page routine for the weekly check. Once we step back, the people running outbound own every piece of it and know how to read it.

Frequently asked questions