Every now and then, our Email Detective data throws up a result that makes me question some of the standard advice we give about email deliverability.
Substack is a good example. We’ve now tracked thousands of Substack emails in Gmail, and have found that 93.1% of them landed in the main (Primary) inbox. Only 6.5% went to Gmail tabs such as Promotions, while just 0.3% ended up in spam.
That gives Substack the highest inbox placement rate of all the marketing email platforms currently included in our Email Detective comparison, and some of the lowest spam rates.
Which is surprising, because Substack isn’t exactly known for sophisticated deliverability tools. But there’s another reason: Substack publishers don’t even send from their own domain.
Instead, every publication sends from an address such as [email protected]. Even paying to connect your own custom domain to your Substack website doesn’t change that. Which goes completely against the usual advice to build a sender reputation around a domain you control.
So why are Substack emails doing so well?
I don’t think the answer is simply that Substack has discovered some deliverability secret the rest of the industry has missed. In fact, the data points to something more interesting.
What our Email Detective data found
Email Detective tracks where real emails actually appear in Gmail, rather than sending test campaigns to a seed list.
Our overall placement database currently contains more than 250,000 tracked emails. To appear in the ESP comparison, a provider needs at least 1,000 tracked messages across at least three qualifying brands, and each brand needs multiple placement observations for that ESP. In short: we have a lot of data to work with.
Looking specifically at marketing platforms, here are some of the current results:
| ESP | Main inbox | Tabs | Spam |
|---|---|---|---|
| Substack | 93.1% | 6.5% | 0.3% |
| Kit | 89.4% | 8.9% | 1.7% |
| Customer.io | 86.5% | 11.1% | 2.4% |
| Beehiiv | 86.4% | 11.5% | 2.2% |
| Mailchimp | 78.6% | 17.6% | 3.8% |
| Omnisend | 73.0% | 22.0% | 5.0% |
| Klaviyo | 67.9% | 31.1% | 1.0% |
Source: Email Detective Gmail Inbox Placement Comparison
As you can see, Substack stands out immediately. But before declaring it the king of deliverability, there’s an important distinction to make.
This isn’t quite the same thing as “Substack has the best deliverability”
When Gmail uses its default tabbed inbox, it separates messages into categories.
“Primary” (or the main inbox) is for emails from people you know, and messages Gmail doesn’t think belong in another category. “Promotions” is a tab specifically intended for deals, offers and other promotional emails. Other tabs include “Updates” and “Social”.
So an email landing in Promotions (or another tab) has not failed to deliver. Gmail has received it and decided that it looks promotional.
For some marketers, landing in Promotions can be a cause for concern. Due to its “secondary” status in the inbox, emails in Promotions tend to have lower engagement rates than Primary emails, and in some cases, can result in 30% fewer opens.
Which is why in this situation, Substack emails seemingly have an advantage compared to, say, Klaviyo emails. Only 67.9% of the Klaviyo emails we tracked landed in the main inbox, versus 93.1% for Substack.
You might initially conclude that Substack has dramatically better deliverability. But here’s the thing: Klaviyo’s spam rate is only 1%. The main difference is that almost a third of Klaviyo messages, 31.1%, land in Promotions.
Which makes sense when you consider that Klaviyo is heavily used by ecommerce businesses. Its customers send sales announcements, discount codes, and other promotional campaigns.
So by putting their emails in Promotions, Gmail is simply doing exactly what it was designed to do.
Substack, meanwhile, attracts a very different kind of sender.
Reason #1: Substack newsletters often aren’t very promotional
If you’re subscribed to any Substack publications, you’ll know what a typical email looks like. It might be an essay about politics, a finance newsletter, commentary on the tech industry, a recipe, a travel story, or someone’s thoughts about running a business.
The purpose is often the content itself, with no further action expected of the recipient once they’ve finished reading it.
An example of a Substack newsletter by fashion blogger Clara Infante that landed in the Primary inbox
Compare that with an ecommerce newsletter whose main objective might be to promote:
20% OFF THIS WEEKEND
or:
Our summer sale starts now
Both are perfectly legitimate emails that subscribers may be happy to receive. But one fits Google’s definition of promotional content much more neatly than the other.
This also helps explain why Kit (another creator-focused newsletter platform) ranks just behind Substack, with 89.4% of messages landing in the main inbox. Beehiiv also had a high number of emails landing in the primary inbox (86.4%).
Inbox placement rates for newsletter platforms like Substack, Kit and Beehiiv from Email Detective
Now, that doesn’t prove creator newsletters automatically get preferential treatment from Gmail. But when the two marketing platforms with the highest main-inbox rates are both strongly associated with creators and newsletters, it’s difficult to ignore the pattern.
My hypothesis is that the kind of email being sent matters enormously here.
Reason #2: People subscribe because they want to hear from a particular person
There’s another difference between many Substack newsletters and conventional marketing lists. With the former, the relationship often starts with the writer.
Someone subscribes because they want to read what a particular journalist, expert or creator has to say. They aren’t necessarily subscribing because they bought a pair of shoes six months ago or ticked a box to receive a discount code.
That potentially creates stronger subscriber intent. Google’s sender guidance places considerable emphasis on sending wanted email, and keeping user-reported spam rates extremely low. (It recommends keeping Gmail spam rates under 0.1% and avoiding rates of 0.3% or higher.)
We can’t see Gmail’s complete decision-making process, and our data doesn’t (yet) allow us to conclude that Substack subscribers are more engaged than Mailchimp or Klaviyo subscribers. But it’s a very plausible part of the explanation.
If someone actively sought out your newsletter because they want your writing, that’s a different relationship from being one recipient among hundreds of thousands on a retailer’s promotional database.
Reason #3: Substack controls the sending setup
This is where things get particularly interesting. Normally, I’d recommend that an established business send marketing emails using a domain or subdomain it owns (e.g. @emailtooltester.com, @news.emailtooltester.com).
But Substack doesn’t give publishers that option.
Even if your publication lives at newsletter.yourdomain.com, the actual newsletter still comes from an address on @substack.com.
That has disadvantages, particularly if you eventually want to move away from Substack and take your sending identity with you. But there’s also a possible deliverability upside: Substack controls the setup.
That removes a surprisingly common source of deliverability problems. It means publishers don’t need to configure the sending domain themselves. They can’t forget to authenticate it correctly. And a brand-new publisher doesn’t suddenly start sending thousands of messages from a sending domain that inbox providers aren’t yet familiar with.
However, I wouldn’t take our results as evidence that shared domains are better than custom domains. Shared reputation can work against you too.
In fact, Substack itself makes this very clear to its users. Its documentation specifically highlights how spam complaints can damage deliverability for its entire creator community, which is why it closely monitors imported lists, and can take action to restrict suspicious ones.
Could that contribute to Substack’s low spam placement rates in our data? Quite possibly. But again, our numbers show correlation, not causation.
Reason #4: Substack isn’t built around relentless marketing automation
There’s another structural difference. Traditional ESPs have become incredibly sophisticated marketing platforms where you can trigger abandoned-cart campaigns, product recommendations, and highly targeted campaigns.
These features are valuable, but they also mean businesses can (and do) send a lot of marketing emails.
Substack’s core email functionality is far simpler. It was built around writing and publishing rather than complex marketing automation. (In our own testing, we found that’s both one of its strengths, and one of its limitations.)
The average Substack newsletter is much more likely to be: “I’ve written something new. Here it is.” That may inadvertently be good for its Gmail placement, as Gmail may simply see less reason to categorize that message as a promotion.
The Klaviyo result may be just as interesting as Substack’s
I don’t think the takeaway from our research should be: “Substack delivers emails better than Klaviyo.” That would oversimplify what we’re measuring.
Klaviyo actually illustrates the difference between deliverability and inbox categorization extremely well. Its 1% spam rate is relatively low in our dataset.
So most Klaviyo emails are reaching Gmail successfully. They’re just much more likely to be filed under Promotions.
And if you’re an ecommerce store emailing someone about a flash sale, arguably that’s exactly where Gmail thinks you belong.
We see a similar pattern with other ecommerce-based platforms. Shopify Messaging has 22.5% of messages landing in tabs but only 1.5% going to spam. Adobe Campaign has 23.4% in tabs and just 0.9% in spam.
Overall, I think this tells us less about an ESP’s deliverability capabilities, and more about the types of messages its customers send.
So, has Substack cracked Gmail?
Probably not. I think we’re seeing several advantages stack up at once:
- Substack content is often editorial rather than overtly promotional.
- Readers frequently subscribe because they actively want to hear from a particular creator.
- Substack controls the sending domain and much of the technical infrastructure.
- It has a strong incentive to police risky email-list imports because reputation is shared across its ecosystem.
- Its relatively simple email feature set means it isn’t primarily used to run high-volume automated campaigns.
Together, those factors could plausibly explain why 93.1% of the Substack messages we’ve observed end up in Gmail’s main inbox.
What marketers can learn from Substack
I wouldn’t recommend moving your ecommerce newsletter to Substack just because you want to escape Gmail’s Promotions tab. Nor would I recommend stripping out every image and button in the hope of making an ecommerce campaign look like a personal letter.
If the email exists primarily to sell something, Gmail may (quite rightly) consider it promotional.
Instead, I think brands can take something more useful from the Substack model:
Not every email you send needs to be a promotion.
And rather than trying to “game” the system to make an email look less like a promotional email, send something people genuinely subscribed to read:
- If you’re a retailer, publish useful buying guides or genuinely interesting editorial content. (It worked for Rare Beauty, who incidentally use Substack for their brand newsletter!)
- If you’re a nonprofit, send an impact story without attaching a donation request to every update.
- If you’re a software company, teach customers something useful rather than putting a sales CTA into every newsletter.
Who knows – a simple shift in your strategy may just get you more visibility in the inbox.
Want more insights like this? We’ll soon be releasing our annual report on ESP inbox performance, based on thousands of observations by our Email Detective Chrome extension. Subscribe to our newsletter to be among the first to receive it.