How Complaint Rates Correlate to Delivered Rates

 Return Path 

John Young, PhD
Director of Product Analytics

In part one of this posting we learned that the Sender Score reputation rank is correlated to delivered rates. So, what about complaints? Are they correlated to delivered rates? Again, yes, there is a strong (negative) linear relationship between delivered rates and complaint rates, which can be seen in the chart below.


Commercial mailers with delivered rates equal to ~90% tend to have much lower complaint rates equal to 0.46%. Whereas, mailers sending from dynamic IPs tend to have delivered rates below 20% with much higher complaint rates around 2.65%.

As we stated in part one of this posting, correlation does not imply causation. However, in the case of complaints – we’re pretty sure that the relationship can be causal, because most large ISPs use complaints as a major factor in determining inbox placement.

So what else impacts your reputation score?

Spam trap hits, inclusion on certain black lists, and high unknown user rates can have negative impacts on your reputation score. For example, when unknown user rates exceed 10%, the average delivered rate plummets by 35%, rejected rates increase as much as 30%; filtered rates jump 10X, and the average reputation score drops 20 points.

“Who” you are and the class of mailer ISP’s consider you to be also affects your reputation score. Commercial mailers (the majority of people reading this blog would fall in that category) have the highest percentage of mail being delivered given the number of messages per IP address.

With fewer spam trap hits, less blacklisting, lower unknown user rates, and fewer complaints per IP, it’s not surprising that commercial mailers have significantly higher delivered rates.

Commercial mailers also have the smallest percent of all the “attempted mail” at ~3%. Meaning, if an ISP sees a total of 100 messages today, only 3 of those messages will come from commercial mailers. Where are most of the other 97 messages coming from? The majority are from dynamic hosts without rDNS records, where a high percentage (although not all) of that mail is spam.

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