I’ll give you a hint — it’s not you. At best, you’re sharing it. And if you’re not careful, you’ll find that a handful of other companies can more or less take it away from you.

Here’s the reality of email marketing in 2019: your house file might have 100,000 addresses on it, or 1M, or 10M, but most of those addresses are actually hosted by one of a very small handful of MBPs.

The Big Three Mailbox Providers

You might be sending a million emails a day, but you’re not sending them to one million different destinations. Rather, you’re sending mostly to Google, Microsoft, Verizon/Yahoo/AOL. In most cases, 80% of the subscribers use one of these three services.

So, while in one sense you own the collection of addresses on the list, three other companies decide whether you can actually send mail those addresses. Google, Microsoft, and Verizon are the gatekeepers. Each one imposes its own arbitrary rules regarding your message frequency, content, engagement rate, and reputation. If you violate any of those rules, these MBPs can — and frequently do — decide to not deliver your email.

For all high-volume email senders, we recommend making a domain distribution analysis of the house file. This will tell you whose deliverability rules you should be worrying about.

Don’t Forget Vanity Domains

It’s easy to count the number of @gmail.com subscribers on your list, but this understates the number of your subscribers who Google controls your access to. Google’s Gmail system hosts many thousands of personal, school, and corporate email accounts — such as berkeley.edu, costco.com, hbo.com, motorola.com, salesforce.com, etc. These school and business accounts should absolutely be included in your domain distribution analysis.

Google is not the only big provider of personalized email domains, of course; Microsoft hosts email for thousands of schools and companies, too: such as Accenture, Adobe, Best Buy, Home Depot, Nike, Sprint, and many, many more.

We can help create a complete domain distribution for your mailing list; this is one of the services we offer.

A Few More Gatekeepers

Depending on the domain distribution, you might find that you need to worry about 3rd-party filters like Cloudmark, which provides spam and malware filtering for Comcast, Cablevision, Time Warner, and many other MBPs, or Proofpoint, which runs filters for Apple domains (me.com, mac.com, icloud.com).

(Proofpoint’s recent acquisition of Cloudmark will presumably result in the consolidation of these two filter technologies. As of mid-2019, they remain separate.)

Consolidation Trends

We’ve been watching Gmail consume more and more market share since its 2004 launch. Today, Google hosts more than 50% of most US-based commercial mailing lists.

Verizon’s recent acquisition of Yahoo Mail and AOL puts it in second place, taking that spot from Microsoft.

Microsoft’s market share has been progressively shrinking, as webmail users abandon Hotmail for Gmail. In terms of deliverability, we find this to be a positive trend. Microsoft’s expectations for for high-volume email senders are opaque. Microsoft’s filters respond very slowly to behavioral changes on the part of senders. This makes it difficult to know what to change, and difficult to test changes.

One critical takeaway is that the deliverability landscape continues to evolve. Maintaining good inbox rates is not a “set and forget” mechanism. Rather, it requires ongoing and proactive maintenance.