Why are your Substack emails going to spam?

Opens dropping, or a subscriber told you they found you in spam? Here are the five real reasons — and a 30-second test that shows exactly where your next post lands.

Forward a test email here

sleeping-seal-2446@in.mailtester.ai

Five real reasons a Substack post lands in spam

Most articles on this hand you a generic checklist. Here's the honest, Substack-specific version: five things decide where your post lands, some of them work differently on Substack than on a normal email tool, and you can stop guessing by testing your actual send.

Cause 1 — Authentication (and what Substack controls)

On Substack, you don't fully control how you're authenticated.

SPF, DKIM and DMARC are how Gmail and Outlook confirm a message is really from who it claims to be. On a standard Substack setup your posts go out from a shared substack.com address, so this part is Substack's to manage — not yours. That's mostly good (Substack keeps it valid), but it also means you can't fix authentication yourself, and you share that sending identity with every other writer on the platform. A custom sending domain (a paid Substack feature) gives you more control. Our report just tells you pass or fail — you don't need to read a single DNS record.

Cause 2 — Engagement (the Substack lever)

Your readers stopped opening and replying — and the filters noticed.

This is the big one on Substack. Mailbox providers watch whether people open, click, and especially reply. A reply is the single strongest 'this isn't spam' signal there is — it's why Substack itself tells you to ask new subscribers to reply to your welcome email. If you grew fast with a giveaway, a cross-promo, or an import, you're carrying readers who never really wanted you: they don't open, some mark you spam, and that drags placement down for everyone. Trimming the dead weight and earning a few replies moves placement more than any subject-line tweak.

Cause 3 — The post itself

A giant header image with little text, a wall of embeds and links, a spammy subject line, link shorteners — filters read all of these as 'looks like bulk marketing.' Substack posts are especially prone to this because they're image-rich by design. You don't have to write like a robot to avoid it; you have to know which specific words and patterns in your draft are costing you. That's the part we'll rewrite for you.

Cause 4 — Frequency & consistency

Going quiet, then posting in a burst.

Going silent for two months and then publishing three posts in a week reads like a sender who can't be trusted. So does suddenly changing how often you send. Mailbox providers reward a steady, predictable rhythm to an engaged audience — which happens to be exactly what builds a Substack readership anyway. If your open rate dropped right after your cadence changed, this is probably part of it.

Cause 5 — Shared Substack reputation

You share a sending reputation with every other Substack.

Because standard Substack posts go out from substack.com, your sender reputation is pooled with the whole platform. If other writers send badly, a little of it can splash onto you — and that's the one cause you genuinely can't fix alone. The honest move isn't to panic: it's to make the things you DO control (engagement, content, consistency) as clean as possible, so you're one of the best senders in the pool, not one of the worst. A custom domain is the only way to stand apart from it.

Most of the time it's not spam — it's Promotions

Before you assume the worst: open Gmail and check the Promotions tab. For a Substack newsletter, that's the more common destination, and it's not a failure — it's delivered, it's openable, your engaged readers still find you. Spam is the filter deciding you might be junk; Promotions is Gmail filing you under 'marketing.' The reasons overlap (engagement, content, image-to-text balance), but the stakes differ. The only way to know which one you're hitting is to look at where a real post actually lands — which is exactly what we measure: your % Primary / Promotions / Spam across Gmail, Outlook and Yahoo.

See where YOUR Substack post actually lands

You can work through that checklist for an afternoon and still be guessing. Or you can forward a post and see it: we open your real email in real Gmail, Outlook and Yahoo accounts and show you the split — Primary, Promotions, Spam — plus the three things most worth fixing first, in plain English, with rewrites for the content ones. The free check gives you the score and the authentication results. No address to sign up with, no card.

Two different tests. Most people mix them up.

If you've ever searched "email tester," half the results check whether the address is valid. We do the other thing — whether the email itself will reach the inbox once you press send.

Address validity

Does this address exist?

You upload a CSV of subscribers. The tool tells you which addresses bounce. Great for cleaning a stale list before a re-engagement send.

Inbox placement

Will Gmail actually show it?

You forward one email — newsletter, launch, client note — to a one-time mailbox. We open it in real Gmail, Outlook, and Yahoo and tell you which tab it landed in, plus what to rewrite before you send it to your list.

Substack Emails Going to Spam? See Where They Land

Why are my Substack emails going to spam all of a sudden? It's rarely one thing. The usual suspects: a recent dip in opens and replies (mailbox providers read low engagement as 'nobody wants this'), a list you grew fast with a giveaway or import, a post that's all image and links, going quiet and then posting in a burst, or readers who never added you to their contacts. Forward your last post and we'll show you which of these is hurting you — ranked by impact.

Can I fix Substack's authentication myself?

Mostly no — and that's the honest part. On a standard Substack setup your posts go out from a shared substack.com address, so SPF and DKIM are Substack's to manage, not yours. A custom sending domain (a paid Substack feature) gives you more control. Either way, the levers you really own are engagement, content and consistency — which is where most of the placement problem actually lives.