If you’ve noticed your deliverability declining in recent years, you’re not alone. Just three years ago, one in six commercial emails failed to reach the inbox. In 2017, that number has jumped to one out of every five. That’s right—worldwide, just 80 percent of commercial email reaches its intended recipient. The news is even worse for brands in the US, where just 77 percent of commercial emails reached the inbox in 2017.

One reason it’s becoming more difficult to reach the inbox is that mailbox providers are constantly changing and improving their filtering algorithms, in an effort to thwart spammers and improve the inbox experience for their users. That’s great news for consumers, but unfortunately some legitimate email gets caught in the crossfire.

Marketers face a tough challenge in overcoming these deliverability concerns, because each mailbox provider is unique when it comes to spam filtering practices, infrastructure requirements, whitelisting and prioritized delivery, sender support, and more. So what works at Yahoo may not work at Gmail—and vice versa.

As a result, marketers need to educate themselves about the specific deliverability rules and requirements of top mailbox providers—or any mailbox provider that constitutes a significant portion of your list. Following are a few tips to get you started, and you can read more in the Marketer’s Field Guide to Gmail, Outlook.com, and Yahoo.

As the world’s largest webmail provider, reaching the inbox at Gmail is critical—and challenging.

With more than one billion active users worldwide, Gmail is far and away the world’s largest webmail provider. So understanding how to reach Gmail inboxes is crucial for marketers—but it’s no easy task. Unlike many of its large competitors, Gmail doesn’t utilize whitelists or disclose use of public blacklists. Historically, Gmail has also been very secretive about its deliverability policies and provided relatively little in the way of support for senders.

The good news is, Gmail has begun to relax the secrecy around its deliverability requirements, starting with the launch of its postmaster site in 2015. Using this site, verified and authenticated senders can access bulk sender guidelines, dashboards, and Gmail’s unique feedback loop. Marketers with a significant number of Gmail subscribers—and let’s face it, that describes most marketers—should get familiar with the content and advice included on this valuable website.

In recent years, we have also learned that Gmail places a great deal of importance on subscriber engagement to make filtering decisions. Engagement metrics look at how users interact with messages from a given sender—things like how many messages are read, replied to, forwarded, and deleted without reading. By analyzing how individual subscribers engage with a given sender’s email, Gmail is able to make more accurate decisions as to which emails are wanted and which should be diverted to the spam folder.

Human judgment is an important component of the filtering process at Outlook.com.

Like most mailbox providers, Outlook.com (formerly Hotmail) considers factors like send volume, spam complaints, and sending practices in making filtering decisions. But they also utilize a unique element in evaluating email content and sender reputation: the Sender Reputation Data (SRD) network. This randomly selected panel of active Outlook.com users provides feedback on whether or not specific messages are “junk,” to help train and improve the Outlook.com filtering algorithm. A “junk” vote will negatively impact a sender’s reputation, while a “not junk” vote is viewed as positive.

In a similar vein, Outlook.com provides its users with a number of inbox management features that help to improve the filtering process. For example, “scheduled cleanup” allows users to set rules to automatically delete old emails and only keep the latest from a sender—which means that over-mailers could be negatively impacted by repeatedly having their email deleted without being read. Another example is Outlook.com’s new “focused inbox,” which separates email into two inboxes. Senders that the user frequently engages with arrive in the “focused” inbox, while the rest is delivered to the “other” inbox—so keeping Outlook.com subscribers engaged is key to having your messages seen.

Outlook.com offers several self-help and escalation options for senders who are experiencing deliverability problems. Senders are first asked to ensure they are following best practices listed on the Outlook.com troubleshooting page. If further assistance is needed, the sender can submit a ticket to the postmaster team.

Whitelists can help to improve inbox placement at both Yahoo and Outlook.com.

Although Gmail does not offer any proprietary or third party whitelisting opportunities, senders who qualify can get priority placement at both Yahoo and Outlook.com. Yahoo offers a proprietary “Bulk Sender Application,” which allows senders to apply for priority placement based on a review of their reputation.

Outlook.com does not maintain a proprietary whitelist, but both Yahoo and Outlook.com participate in Return Path’s Certification program. This certified whitelist provides preferential treatment to accredited senders, allowing them to bypass certain filters and reach the inbox more consistently. Those accepted into the Certification program are required to meet and maintain stringent standards for sending practices, email infrastructure, and more.

About the Author: Jen Ribble is a marketing professional with more than 15 years of experience in the tech and financial services sectors. She currently leads PR and content marketing efforts for Return Path, the world’s leading expert in email deliverability.