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- 📩 More co-reg, more spam complaints
Most deliverability postmortems get written once the outcome is known.
This one is still open, which is why I think it is worth your time.
A publisher I follow runs several newsletters. Recently they increased the number of co-reg subscribers going into one of them.
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Here is what happened, in order.
The change. Co-reg volume went up.
The damage. Gmail's reported spam rate stayed above 0.30% for four consecutive days, with a high of 0.49%.
The alert. Their monitoring dashboard flagged the trend, and Google's compliance report marked the user-reported spam rate as needing work.
The response. They cut incoming co-reg volume in half.
The next reported day. 0.06%.
It would be easy to end the story there. They are not ending it, and neither would I.
Everything else passed
The detail I keep coming back to is what was not wrong.
The other checks in that compliance report, including authentication and one-click unsubscribe, came back compliant.
So the technical setup was fine while complaints ran well past Google's limit.
A clean setup tells you the email was sent properly. It says nothing about whether the person receiving it wants it.
What more co-reg does to your list
Co-registration lets someone subscribe to your newsletter while signing up through another site or partner.
It can bring in a steady flow of subscribers. Increase that flow and you also change who is receiving your emails, because more of them are new to you and still deciding whether they want you at all.
So walk through what those people experience.
Did they understand what they signed up for?
Do they recognize your sender name?
Does the newsletter match the promise they saw?
Those questions matter at any volume. Push acquisition harder and any weak answer reaches more people.
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The number your ESP will not show you
Check which complaint rate you are actually reading.
Your ESP's standard complaint report does not tell you Gmail's complaint rate. Yahoo sends individual complaint reports through its feedback loop. Gmail does not give your ESP that same feedback.
So you can look at a low complaint rate in your ESP while Gmail subscribers are hitting spam.
To see Gmail's number, you need Google Postmaster Tools, directly or through a platform connected to that data. Even then, you get aggregate reporting. Google will not tell you which subscribers complained.
Google's guidance is to stay below 0.10% and never reach 0.30%.
I would not get comfortable at 0.29% just because it clears the higher line. You want room for variation, especially while you are increasing acquisition.
Why they changed one thing
They did not tighten their Base Sending Segment in that first move. They did not touch the newsletter either. They pulled back the thing that had changed and started watching.
Change acquisition, frequency, and segment windows all at once, and you will have a hard time knowing which one helped.
Cutting acquisition in half is not the answer to every complaint spike. It fit this case because acquisition was what had moved.
For you, it might be a new source, a broader sending segment, or more frequent sends.
The useful question is the same: what changed before complaints started climbing?
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One good day is one good day
The drop to 0.06% is the direction you want. It does not prove the problem is solved, and it does not prove how much of the improvement came from the cut.
Google also notes that a better spam rate can take time to show up as better spam classification. So even a good number does not mean your inbox performance has fully recovered.
What comes next is less exciting. They want to see the lower rate hold across more reported days, and they are watching clicks and how incoming subscribers respond.
Acquisition stays down for now, and that has a real cost: slower growth, accepted on purpose. An alert only helps if you act on it while there is still a decision to make. Seeing the warning and carrying on at the same volume would have fixed nothing.
Run this on your own list this week
Line up two weeks of data. Put your Gmail spam rate next to your acquisition activity.
Mark every change. Note when you increased volume, added a source, or changed who receives your newsletter.
Investigate the change first. If complaints rose around the same time, look there before you blame the subject line or the content.
Write down your first adjustment and its date. That record is how you judge the reported days that follow.
Sign up through your own co-reg path. Make sure a new subscriber can tell which newsletter they are joining and what they will receive.
Then reply and tell me what changed right before your last complaint spike. I want to know how many of you can actually name it.
GM.