Email Deliverability: Inbox Providers Breakdown

Klaviyo Email Deliverability and inbox providers

Editor’s note: We provide advanced email marketing workshops for Klaviyo customers throughout the year to teach topics from advanced email segmentation to automated email series to the state of the industry. The following video and transcript are an excerpt from Jake’s presentation on list growth and management at our New York City workshop.

There are two big categories of inbox providers and how they think about filtering email. There’s the older ones and the newer ones.

Older inbox providers: Yahoo, AOL, and other

The older ones are AOL, Yahoo and other – which are .edus, .orgs, stuff like that. They rely on third party services to handle their filtering. The way that they work, the way they think when a message comes in – is this good or bad, should I put it in the inbox or the spam folder – is they primarily look at IP reputation.

The reason they use IP reputation means going back to how email worked in the ’90s, which is when they were building all of these filtering systems. In the ’90s, the way that you would set up email is you’d have a box in a closet (a server), and it would be attached to an IP address, which is used to send the mail. If you wanted to change the IP address, that was hard. You needed to buy a new box or reconfigure the box that you had. So it didn’t change very often.

But as technology got more sophisticated, we had virtual servers. People got more familiarity with how to do this stuff, and the IPs would change frequently. As a result of that, having the one thing that you could look at – where is this mail coming from so I can determine if this is a bad sender versus a good sender – became more difficult to track.

These guys index heavily on IP reputation. They look at all the mail coming from an IP, and they say, like, “Do people open this or not?” If not, then it must be bad, right? They mark it as spam. It must be bad. In bulk. The other thing they do is they rely on blocklist providers who will tell them who is bad.

Newer inbox providers: Gmail and Hotmail

Eventually, Gmail and Hotmail came around, and started to think about things differently. The way they think about it – this is important for you guys to understand – who is Gmail’s customer? Everybody. It’s your customer, right?

But Gmail serves a different purpose for that customer. They are filtering down the crap you get so that you only see the stuff that you want. It’s fundamentally at odds with what you’re trying to do, which is tell your customer stuff they care about.

So they have come up with new rules and new ways to determine if someone should see your memo. They do not rely on IPs at all, because IPs are really easy to switch up now. It’s not a reliable way to determine if this person sending the message is still the person that’s sending this new message.

There’s a more reliable and consistent way, which is sending domains.

So does anyone have a dedicated sending domain? No? Okay we can show you how to do that. So the way that works is, if you want to send mail, you have an IP address and a sending domain. The sending domain requires a record that you put on your website, which means that you have to own the website. That’s much harder to bait or to switch than an IP address. So Gmail goes, “Oh, cool. If they have the same sending domain, that’s unlikely to switch very often. Therefore, we should really rely on that more as a signal as to whether or not the sender is good.”

So Gmail does not really look at IP reputation. They really look at sending domain, because they care about your brand and how you are measuring across all your recipients. Then they go to recipient engagement. They have two level filters:

  • Top level:  hey, is this brand any good?
  • Next level: if we decide the brand’s good, does this person who’s receiving your messages actually open it all the time? If they do, cool. They want it. We’ll put it in the inbox. If they don’t, cool, they don’t want it. Guess where it goes. Spam filter, yeah.

Who uses Gmail personally? First of all, this is working if everyone uses Gmail. Second of all, when was the last time you opened the spam folder in your personal? Have you noticed it’s all legitimate brands? It’s real businesses. It’s not crap. This is why. This is why. You don’t open those emails, so Gmail goes, “Cool, you don’t want it,” and they stick it in the spam folder.

Here’s how this works. Hotmail is one step in the right direction. They care a lot about recipient engagement, but in a different way. Instead of thinking about it in, like, a promotive sense – like if you open, we will then put it in the inbox – they use this to penalize you. If you send to someone who does not open an email in six months to a year, oh my goodness. They’re, like, “Cool, they’re not cleaning their list. Spam folder. See ya.”

The thing you have to remember with Hotmail is they use the same logic from Outlook, and Outlook serves business customers. So they’re really, really focused on helping 1:1 conversations rise to the top, and one to many conversations to go away. Your open rates will definitely be lower with Hotmail than any other provider.


To see Jake’s entire presentation on managing contacts, collecting new leads, and who to target, visit the Klaviyo Help Center List Building and Management page.

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