Sender reputation: what it is built from, how to check it and how to recover

Sender reputation decides whether a mail server accepts your message, puts it in the inbox or files it as spam. There is no single score. Gmail, Outlook, Yahoo and every other provider keep their own, built from their own data, and none of them publishes the full formula.

What they do publish is what the scores rest on. This page sticks to what providers and sending services say themselves, with links at the end.

What sender reputation is built from

Spam complaints

The harshest signal. Google advises keeping the complaint rate below 0.1% and never reaching 0.3%; Yahoo sets its bar at 0.3%. Amazon SES puts an account under review at a 0.1% complaint rate and may pause sending at 0.5%. One detail from Google: the rate is measured against messages delivered to engaged recipients' inboxes, so mail that went straight to spam is not part of the calculation.

Mail to addresses that do not exist

Those messages come back as bounces and show that the sender does not look after the list. Amazon SES recommends keeping the bounce rate under 2%, reviews accounts at 5% and may pause them at 10%. Google's advice includes automatically unsubscribing recipients after several bounces.

Spam traps

Addresses nobody signed up with. Some used to be real mailboxes that sat unused and bounced for a long time. Hitting a trap tells the provider the list was collected or cleaned badly, and Amazon SES notes that even a small number of hits can hurt your reputation badly.

Authentication

Without SPF, DKIM and DMARC a provider has nothing to tie your history to. Gmail requires SPF or DKIM from everyone, and from senders of around 5,000 messages a day or more it requires SPF and DKIM together, a DMARC record and domain alignment. See the Gmail sender requirements and the setup guide.

When volume jumps too fast

Google warns that without a history of large sends, sudden spikes are risky, and even abruptly doubling your previous volume can lead to rate limiting or a reputation drop. A new domain or IP is warmed up gradually.

Engagement and blocklists

Google suggests considering unsubscribing people who do not open or read your mail, and Amazon SES asks senders to stop mailing them. Then there are blocklists. If your IP or domain is on a Spamhaus list, some servers will refuse the message before they look at anything else.

IP reputation vs domain reputation

Providers rate both. Google even counts limits separately: the DKIM and SPF quotas belong to your domain, while an IP address quota is shared by everyone who sends from that address.

A shared IP at a sending platform serves many customers. Google puts it plainly: the activity of any sender on a shared IP affects the reputation of all of them. When you pick a platform, look at how strictly it removes customers with bad lists.

A dedicated IP removes the neighbours, but the whole history is yours and the address has to be warmed up. It pays off with steady volume that gives providers enough to learn from. For small or irregular campaigns a good platform's shared pool is usually the safer choice.

So which matters more? For most senders the question answers itself. The IP belongs to the sending platform, while the domain stays with you through every move, and that is the history you build over years and cannot escape by switching tools. If you send different kinds of mail, Google recommends keeping them apart: one From address per category, and if you use several IPs, one message type per IP. Keep in mind that Gmail counts subdomains together with the primary domain toward the 5,000-message threshold.

How to check your sender reputation

• Google Postmaster Tools. Spam rate, authentication, encryption, delivery errors and your compliance status against Gmail's requirements. The domain and IP reputation dashboards (High, Medium, Low, Bad) are being retired and are not part of the new Postmaster Tools. Google's own reasons: the data is hard to act on, it reflects changes slowly, and it misleads because reputation is only one of many factors. Setup is covered in the Postmaster Tools guide.
• Microsoft SNDS. Per-IP data for Outlook.com. You sign in with a Microsoft account and request access to the IPs you are responsible for. The Junk Email Reporting Program sends reports when users mark your mail as junk.
• Sender Score. A 0 to 100 score for an IP address from Validity. Complaints, mail to unknown users and spam traps are among the factors Validity lists, and its blog calls senders scoring 91 and above best in class.
• Blocklists. Check your domain and IP with our blocklist checker. A homemade check through public resolvers does not work for Spamhaus: via 1.1.1.1 it returns a refusal code, and via 8.8.8.8 it may return nothing, so a listed address looks clean.
• Your own headers. Send yourself a message at Gmail and Outlook and read the Authentication-Results line in the header analyzer. A failing SPF, DKIM or DMARC shows up right away.

No tool shows the whole picture. Postmaster Tools covers Gmail only, SNDS covers Outlook.com only, Sender Score covers IPs only. The most direct warning sits in your own reports: opens drop at one provider while bounces or complaints rise. More on why emails go to spam.

Our advice is to set up Postmaster Tools and SNDS before anything goes wrong. You will learn what normal numbers look like for your own list, and when something breaks you will have a baseline to compare against.

What to do when your reputation drops

1. Find when and why. Which campaign started it, at which provider, and what changed: a new segment, template, volume or sending service. Bounce codes help a lot. Gmail, for instance, answers 4.7.27 for an SPF problem and 4.7.30 for a DKIM problem, while 5.7.1 usually means a policy rejection, so the issue is the sender.
2. Narrow sending to active recipients. While you recover, mail people who opened and clicked recently. Inactive subscribers get a separate, small re-engagement send first, and only then a decision on whether to keep them.
3. Clean the list. Remove hard-bounced addresses and everyone who complained. Amazon SES says to drop complainers immediately and not to send them even an unsubscribe confirmation. Validate the rest so dead mailboxes do not cause a fresh wave of bounces.
4. Check authentication. Every service that sends on behalf of your domain has to be in SPF and sign with DKIM. If DMARC has sat at p=none for a long time, move toward quarantine, starting with the t=y flag.
5. Deal with blocklists. Fix the cause first, then request removal. The steps for the major lists are in getting removed from a blacklist.
6. Bring volume back gradually. No sudden jumps, as in a warm-up, and watch complaints after every send.

Providers do not publish recovery timelines. Google's only official number concerns Gmail's support rather than reputation: a bulk sender above a 0.3% complaint rate loses mitigation and becomes eligible again after seven consecutive days below that rate.

What not to do

• Switch domains. A new domain starts with no history, and Gmail applies enforcement to new domains on a faster schedule. If the list stays the same, the problem moves with it.
• Move to another sending platform without cleaning the list.
• Buy lists or "pre-warmed" IPs. Not buying or renting addresses is the first item in the Amazon SES advice.