How marketers can navigate mailbox provider reputation assessments

Guy Hanson, senior director, professional services, Return Path, explains how marketers can ensure their emails are reaching their subscribers’ inboxes.


Marketers around the world rely on email, but when email fails to reach the inbox, brands lose the opportunity to connect with customers and ultimately make a sale. Year after year, research shows a clear link between reputation and deliverability so it’s critical that sender reputation is monitored and maintained at the highest possible level.

For major mailbox provider’s (MBPs), the main goal is to deliver the messages their users want while keeping unwanted and potentially damaging content from their inboxes. Many MBPs take a multi-layered approach when assessing ‘good’ and ‘bad’ emails.

Firstly, they identify a sender’s reputation to judge whether mail from that sender is legitimate and wanted by their users. Reputation is calculated using a variety of metrics, and each MBP will use their own formula to judge whether or not to accept incoming mail and where to filter it.

They will then typically perform authentication checks, making sure the mail meets any published Sender Policy Framework (SPF), DomainKeys Identified Mail (DKIM) or Domain Message Authentication Reporting and Conformance (DMARC) records, which provide greater assurance of the sender’s identity.

Finally, they will take a more detailed look at what content is being sent and how the subscriber engages with it. It’s therefore worth noting that if a sender fails at the first check, it’s likely their mail will not be accepted by the MBP, let alone whether it gets placed in the inbox of spam folder.

Identifying sender scores

One way marketers can find out how reputable they are as a sender is by using . Similar to a credit score, a Sender Score is an evaluation of sending practices that determines the quality of marketers’ messages and overall email programme. Determining message and email programme quality with a rating from 0 to 100, Sender Score uses data points and algorithms similar to what MBPs use to give an accurate representation of how they view emails. It works in the same way that a person with well-managed finances will have a high credit rating. If a marketer looks after its email reputation by ensuring they have healthy metrics – i.e. low complaint rates, good data quality, high subscriber engagement and minimal industry blacklists – then their Sender Score will be high.

In 2017, senders scoring above 90 had an average delivered rate of 91%. Meanwhile, those who fell one band below (81-90 in Sender Score rating) saw far less mail delivered, with an average of 68%. The remaining senders – those scoring between zero and 80 – saw more than half of their messages rejected at the MBP gateway. While not good news for lower quality senders, it shows that MBPs’ filters are working as intended. Senders with a poor reputation and questionable sending practices are being successfully blocked from users’ inboxes.

Reputation metrics

While each MBP determines sender reputation using their own formula, there are some common metrics that all weigh heavily into their calculations. Among these are complaints, spam traps, unknown users and engagement.

A complaint is generated when a mailbox user marks a message as junk or spam. A high complaint rate signals to mailbox providers that the sender’s content is unwelcome and should be sent to spam. In 2017, senders scoring above 90 were the only ones who managed to maintain a complaint rate below 1%. Those with a Sender Score between 11 and 90 had an average complaint rate ranging from between 2.8% and 3.6%, while senders scoring below 10 had an alarmingly high average complaint rate of 7.4%. Most MPBs recommend no higher than a 0.2% complaint rate. Best in class senders should be aiming for <0.1%.

An unknown user is an email address that never existed, has been terminated by the mailbox provider, or was abandoned by the mailbox user. Mailbox providers will return a hard bounce code (5xx) indicating when email is sent to an unknown user. Senders who not only retain unknown users but send to a high percentage of them are perceived by mailbox providers as having poor list hygiene practices, which will impact on their sender reputation. Senders scoring above 90 tend to keep relatively clean lists, with average unknown rates of 1%.

Spam traps are email addresses that don’t belong to active users and are used to identify both spammers and senders with poor data quality practices. There are two types of spam traps: pristine and recycled. Pristine spam traps are created solely to capture bad mailers. These email addresses were never owned by a real person and therefore shouldn’t be on any marketer’s list. Recycled traps are addresses that were once held by a user but have been abandoned and converted into spam traps. This year, senders scoring above 90 were the most successful at keeping their lists clear of spam traps with an average of 0.36 spam traps hit.

Finally, subscriber engagement is taken into account – albeit differently depending on the MBP. For years, the likes of Gmail, Microsoft, Yahoo, and AOL have been considering the way subscribers interact with their emails and using this to help determine deliverability. Of the four mailbox major providers, inbox placement decisions at Yahoo, Microsoft, and Gmail showed a strong correlation with Sender Score in 2017. Senders scoring below 60 see less than half their email placed in the inbox at these three providers. Yahoo is a little more lenient, allowing senders scoring as low as 81 to exceed the global inbox placement rate (80%).

Sender reputation by industry

When comparing different industries, there are some that stand out more than others when it comes to email reputation. By analysing Sender Scores and Inbox Placement Rates of over 17,000 commercial senders, the travel industry comes out on top, with a Sender Score of 93 and an Inbox Placement Rate of 89%. This could be because their end objective tends to involve a high value transaction which means they take greater care at ensuring their emails are reaching their recipients. Travel emails are also inspirational – people always like to think of going on a nice holiday, especially in the depths of winter, which can make these emails much more appealing to open.

Following the travel sector is the toys, hobbies and crafts industry, then the telecommunication sector. Interestingly, all industries either maintained or increased their average Sender Score in 2017, with the technology/software/internet industry seeing the greatest increase (11 points). One reason for this could be the implementation of GDPR, regulations that have forced businesses to improve their email marketing practices, meaning lower complaints and higher engagement. Indeed, in our recent Deliverability Benchmark report, email deliverability rose to an average inbox placement rate (IPR) of 86% – just above the global average – which could be a reflection of these regulations.

As email marketing continues to evolve, MBPs constantly adapt their acceptance criteria in turn, to identify which senders should be sent to the inbox, and which should be filtered to the spam folder. Email reputation is a delicate ecosystem which requires persistent supervision and refinement. It’s vital that email marketers do not underestimate the impact of factors such as subscriber complaints, unknown users and spam traps. Marketers require full insight into their customers’ email journey – and how their emails are being perceived at each point of that journey – in order to ensure their emails are reaching their subscribers’ inboxes.

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