The Signal Game

Every mailbox provider gives each of your emails a hidden score, and that score decides whether it reaches the inbox. This article explains what goes into the score, which parts count most, and how Gmail, Microsoft, Yahoo and Apple differ. It ends with what good senders do to keep their score high.

The hidden scorecard

A mailbox provider (the company that hosts the reader’s inbox, such as Gmail) never shows you the score. It isn’t a simple pass or fail. The provider combines many signals with your history, and the result can be different for every reader.

What goes into the score

Providers group signals into four kinds.

Who you are (identity):

• Your sending IP address (the internet address of your mail server) and its history.
• Your sending domain and its authentication records (SPF, DKIM and DMARC, which prove the email is really yours).
• Your server’s name and reverse DNS (a record that turns the IP address back into a name).
• Whether the hidden sender address and the visible From address use the same domain.

What you’ve done before (reputation):

• How often people marked your emails as spam.
• How many of your emails bounced (came back undeliverable).
• How people engaged with your past emails.
• Whether you are on a blocklist (a public list of known spam sources).
• Whether you hit spam traps (addresses that exist only to catch senders with bad lists).

What you’re doing now (behaviour):

• Today’s volume compared with your usual volume.
• When you send.
• Who is on the list: lots of new or long-silent addresses look risky.
• Sudden changes in content or layout.

How readers respond (engagement):

• Opens, clicks and replies.
• Spam complaints and unsubscribes.
• Emails received but never touched.

Which signals count most

Providers keep their exact formulas secret, so nobody can rank these precisely. Most experienced senders agree on this rough order.

1. Spam complaints. This is the strongest bad signal almost everywhere. Google asks senders to keep their spam rate below 0.1% (1 complaint per 1,000 emails) and never reach 0.3%. Yahoo uses the same 0.3% limit, and Microsoft also counts the Junk reports it gets.
2. Engagement. Gmail’s filters ask “will this reader want this?”, not only “is this spam?”. If the answer looks like no, the email drifts toward spam, even when everything else is fine.
3. Reputation. Your domain’s track record over weeks and months. Each new email starts from this baseline.
4. Authentication. Passing SPF, DKIM and DMARC is expected. Failing is a clear bad sign. Gmail, Yahoo and Microsoft require all three from bulk senders.
5. Content and layout. Links, HTML and wording. They matter less than the items above, but can tip a close call.
6. Infrastructure. Your IP address’s reputation, how your server connects, and TLS (encrypting the connection).

How each provider differs

Each provider builds its own filters. What practitioners see in practice:

Gmail

• Engagement first. If a reader keeps ignoring you, Gmail moves your email toward their spam folder, even if your overall numbers look fine.
• Machine learning. The filters learn from huge amounts of mail and from what users do. They catch patterns that fixed rules miss.
• Per-person decisions. Two Gmail users can see the same email in different folders.
• Tabs are not spam. The Promotions and Updates tabs are normal inbox placement. Email there is still seen, just less often than in the main tab.
• Postmaster Tools. Google’s free dashboard shows your spam rate and authentication results.

Microsoft (Outlook.com, Hotmail, Live)

• IP reputation counts more. Microsoft is known to weigh your IP address’s history heavily. A new IP gets close checks.
• SNDS (Smart Network Data Services) shows how your IP addresses behave at Microsoft, including complaints and spam trap hits.
• JMRP (Junk Mail Reporting Program) is Microsoft’s complaint feedback loop. It sends you a report when someone marks your email as junk.
• Stricter on content. Senders often find Microsoft’s content filters tougher than Gmail’s.
• Slow to recover. A reputation drop at Microsoft often takes longer to fix than at Gmail.

Yahoo and AOL

Yahoo and AOL share the same filtering systems.

• Complaint Feedback Loop. Yahoo’s feedback loop sends you complaint reports. Sign up and act on them.
• Watches volume. Sudden jumps from unfamiliar senders get extra checks.
• Slows senders down. When your traffic looks suspicious, Yahoo delays it with a 421 “try again later” reply (a soft bounce: a temporary failure your server retries).
• Takes authentication seriously. In 2014 Yahoo set a DMARC p=reject policy on its own yahoo.com addresses. Mailing lists that resent Yahoo users’ email were suddenly rejected. It was the first big provider to do this.

Apple (iCloud Mail)

• Mail Privacy Protection. The Apple Mail app can load every email’s images in advance, including tracking pixels (tiny images that report an open). So opens from Apple Mail users look like opens even if nobody read the email. This affects any account read in Apple Mail, not just iCloud.
• Few sender tools. Apple offers almost nothing for checking your reputation or placement.

How reputation is built

Reputation isn’t one number. Providers keep separate records for your domain and for each IP address.

Domain reputation

Built from:

• The share of your emails marked as spam.
• The share that bounce.
• Opens, clicks and replies across all your readers.
• Spam trap hits.
• How steady your volume is.
• How long your domain has existed.
• Whether SPF, DKIM and DMARC are set up correctly.

Domain reputation follows you. If you switch ESP (the service that sends your email) or IP address but keep your domain, your reputation comes with you, good or bad.

IP reputation

• New IP: no history, so no trust. You must warm it up (raise volume slowly).
• Shared IP: used by many senders, so it carries all their reputations. One bad sender can hurt everyone.
• Dedicated IP: used only by you, so it carries only your reputation. You get full control and full responsibility.
• IP pool: a group of IP addresses an ESP spreads mail across. The whole group’s behaviour sets its reputation.

How the two work together

• A new domain on a new IP starts with no trust at all. Warm both up slowly.
• A known domain on a new IP often gets some grace while the IP warms up.
• A new domain on a good IP is still checked closely, because the domain has no history.
• A good domain on a blocklisted IP still runs into trouble.
• The strongest position is a trusted domain on a trusted IP.

Tricks don’t last

Some senders try to fool the filters:

• Switching domains again and again to escape a bad reputation.
• Paying for “warming” services that fake opens and replies.
• Hiding the unsubscribe link.
• Adding invisible text to fool content filters.
• Buying lists and deleting the addresses that bounce.

These may work for a while. They always fail in the end:

1. Filters keep learning. A trick that works today gets caught tomorrow.
2. Real engagement catches up. Without people who want your email, your reputation sinks.
3. Getting caught is costly. A blocklisting, a burned domain or a ban from your ESP costs far more than doing it right.

How good senders play

They only email people who want it. Who is on your list is the thing you control most. Each email goes to someone who signed up, expects it and has engaged recently.

They send as often as promised. Daily emails to someone who signed up for weekly ones lead to complaints.

They watch and react. They track complaints per send, bounces per list segment and engagement per provider. When a number dips, they look into it the same day.

They treat readers differently. Active readers get more email. Inactive readers get a win-back email, and if they still don’t respond, they are removed.

They warm up patiently. A new domain or IP is warmed over weeks, not days. They start with their most engaged readers.

They keep streams apart. Receipts and password resets go out on a different subdomain or IP from marketing. A marketing problem can never block a password reset.

How to estimate your score

You can’t see the score, but you can get close.

Provider dashboards

• Google Postmaster Tools: spam rate and authentication results at Gmail.
• Microsoft SNDS: IP behaviour, spam trap hits and complaints at Outlook.com.
• Yahoo Complaint Feedback Loop: complaints from Yahoo users.
• Blocklist checks: look yourself up on Spamhaus, Barracuda and other major lists regularly.

Warning signs in your own numbers

• Opens slowly falling. More of your email may be going to spam.
• More soft bounces. Providers may be slowing you down because of your reputation.
• A sudden drop at one provider only. That provider has lowered your score.

Remember that Apple’s privacy feature inflates opens, so read open rates with care.

Tests you can run

• Inbox placement tests: send to test accounts at each major provider and see where the email lands. The Mailwel test inbox does this.
• Header checks: look at the Authentication-Results header to see whether SPF, DKIM and DMARC passed. The header analyzer reads it for you.
• Spam score headers: some receiving systems add their own score, such as SpamAssassin’s X-Spam-Status.

What to do next

• Check your SPF, DKIM and DMARC records with DNS lookup.
• Send a test email to the test inbox and see your results.
• Sign up for Google Postmaster Tools, Microsoft SNDS and JMRP, and Yahoo’s Complaint Feedback Loop.
• Keep your spam complaint rate below 0.1%.
• Remove readers who haven’t engaged in months, after one last win-back email.