What Is Sender Reputation?

Sender reputation is the judgement inbox providers form about whether mail from you is worth delivering to the inbox.

It is not a single published number. Gmail, Microsoft, Yahoo and corporate filters each maintain their own view, none of them show it to you directly, and they weight the inputs differently. What they share is the inputs.

Domain reputation and IP reputation are different things

The distinction that causes most confusion, so it goes first.

Domain reputation attaches to the sending domain. It is built from that domain’s sending history and it follows the domain wherever you send from. Change platforms, change infrastructure, change providers, the domain keeps its reputation.

IP reputation attaches to the sending IP address. It is built from that IP’s traffic. Move to a new IP and you start unknown.

For most cold senders, domain reputation carries more weight. Gmail in particular leans heavily on domain-level signals for senders who are not at enormous volume. This is why a fresh dedicated IP attached to a damaged domain changes nothing, and why keeping your domains through a migration matters more than which tool you send from.

The six signals that build it

1. Complaint rate. Recipients marking you as spam. The most heavily weighted signal by a distance. Keep it below 0.1%.

2. Bounce rate. Hard bounces to non-existent addresses signal a poor list. Keep it below 2%, and treat 3% as the point where damage begins.

3. Engagement. Opens are unreliable since image pre-fetching became default, but replies, forwards, and mail being moved out of spam are strong positive signals. Deletion without opening is a weak negative one.

4. Volume consistency. Steady sending looks like a business. A domain that sends 50 a day for a month then 3,000 on a Tuesday looks compromised.

5. Authentication. SPF, DKIM and DMARC passing and aligned. Not reputation exactly, but failures are weighted as a strong negative. SPF, DKIM and DMARC explained covers the setup.

6. Spam trap hits. Addresses that exist only to catch poor list practice. One hit is a serious negative signal and it implies your sourcing is bad.

Notice what is absent: your copy. Content matters far less than any of the above, which is why rewriting subject lines rarely fixes a placement problem.

How to check yours

No provider shows you a single score, but four things get you close.

Google Postmaster Tools. Free, and the closest thing to direct feedback. Reports domain reputation, IP reputation, spam rate and authentication success for mail to Gmail. Needs a few hundred messages a day to the same domain before data populates. If you send meaningful Gmail volume and have not set this up, do it today.

Microsoft SNDS. The Outlook equivalent for IP data. Less rich, still useful if you send volume to Microsoft-hosted mailboxes.

Inbox placement testing. Does not report reputation directly but tells you the outcome, per provider, which is what you actually care about. How to run one.

Blocklist checks. Public blocklists are not the same as provider reputation, but a listing is both a symptom and a cause.

Third-party “sender score” tools exist. Treat them as directional, they are modelling the same inputs you can check yourself, not reading a provider’s internal score.

What damages it, in order

1. A bought or unverified list. Bounces and spam traps together, immediately.
2. A volume jump. Going from warmed to full throttle in a week.
3. Poor targeting. Complaints come from people for whom your message makes no sense.
4. Broken authentication. Particularly an SPF record over the ten-lookup limit, which fails silently.
5. A shared tracking domain. Your links carrying every other customer’s reputation.
6. Sending into disengagement. Continuing to mail people who never open anything.

What rebuilds it

Slowly, and in this order.

Stop the cause. Obvious, routinely skipped. Pause the campaign producing the complaints before doing anything else.

Clean the list. Re-verify everything. Remove anyone who has not engaged in six months.

Reduce volume, then ramp. Drop to perhaps 20% of previous volume and rebuild over three to four weeks.

Fix authentication. Check rather than assume.

Send mail people reply to. Engagement is the only positive signal you directly control. A smaller, better-targeted campaign rebuilds reputation faster than a large careful one.

Expect weeks, not days. Reputation is built from sustained behaviour and it is repaired the same way.

The practical consequences

Keep your domains. They carry the asset. If an agency or vendor registered your sending domains, the reputation you built is theirs.

Separate your sending domain from your primary one. A damaged cold email domain should never be the domain your invoices come from.

Three mailboxes per domain, five at most. Concentration means one problem takes out everything on that domain.

Watch complaint rate above all else. It is the signal providers weight most heavily and the one most within your control, because it is downstream of targeting.

The short version

Sender reputation is a provider’s judgement about your mail, built mostly from complaint rate, bounce rate, engagement, volume consistency and authentication. Your copy barely enters into it.

Domain reputation matters more than IP reputation for most cold senders, and it follows the domain rather than the tool, which is why owning your domains is the single most valuable thing you can do about it.

Check Google Postmaster Tools, run placement tests, and if reputation is damaged, stop the cause first, clean the list, drop volume and rebuild over three to four weeks.

Next: the deliverability guide, or shared versus dedicated IPs.

Frequently asked questions

What is sender reputation?

The judgement inbox providers form about whether your mail belongs in the inbox, built from complaint rate, bounce rate, recipient engagement, volume consistency, authentication and spam trap hits. No provider publishes it as a single score.

What is the difference between domain and IP reputation?

Domain reputation attaches to your sending domain and follows it wherever you send from. IP reputation attaches to the sending IP and resets when you move. For most cold senders, domain reputation carries more weight.

How do I check my sender reputation?

Google Postmaster Tools for Gmail data, Microsoft SNDS for Outlook, inbox placement tests for per-provider outcomes, and blocklist checks. No single number exists.

How long does it take to repair sender reputation?

Weeks rather than days. Stop the cause, clean the list, drop to around 20% of volume and ramp back over three to four weeks while sending mail people actually reply to.