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Email Deliverability on Substack: How to Keep Your Newsletter Out of Spam

Best Practices

You can write the best newsletter in the world, but it doesn’t matter if it lands in spam. Email deliverability — the percentage of sent emails that actually reach the inbox — is the invisible foundation of every newsletter business.

Substack handles most deliverability infrastructure for you, but there are meaningful actions you can take to improve where your emails land.

How Email Deliverability Works

When you hit publish on Substack, your email doesn’t go directly from Substack’s servers to your subscriber’s inbox. It passes through multiple checkpoints:

  1. Substack’s sending infrastructure prepares and sends the email
  2. The receiving email server (Gmail, Outlook, etc.) checks the sender’s reputation
  3. Spam filters analyze the email’s content, formatting, and metadata
  4. Inbox sorting determines placement: Primary, Promotions, Updates, or Spam

Each step can redirect your email away from the Primary inbox. Understanding what influences each step helps you optimize.

What Substack Handles

Sending Infrastructure

Substack manages its own email sending infrastructure, including:

  • SPF and DKIM authentication: email authentication protocols that verify the sender
  • Dedicated IP reputation: Substack maintains sending IPs with good reputation
  • Bounce handling: invalid email addresses are automatically managed
  • Unsubscribe compliance: one-click unsubscribe headers (required by law and email providers)
  • Volume management: emails are sent in batches to avoid triggering rate limits

Custom Domain Email Authentication

If you’ve set up a custom domain, Substack configures email authentication (SPF, DKIM) for sending from your domain. This can improve deliverability for subscribers whose email filters weigh domain authentication heavily.

What You Control

Subject Lines

Email providers analyze subject lines as a spam indicator. Avoid patterns that trigger filters:

Risky patterns:

  • ALL CAPS: “HUGE ANNOUNCEMENT TODAY”
  • Excessive punctuation: “You won’t believe this!!!”
  • Spam trigger words: “free money,” “act now,” “limited time offer”
  • Misleading urgency: “RE: Your account” (when it’s not a reply)

Effective patterns:

  • Clear and descriptive: “3 Trends Reshaping Remote Work”
  • Specific: “Why Apple’s Q4 Earnings Beat Expectations by 12%”
  • Consistent with your brand: your regular readers should recognize the style

Content Quality

Email providers increasingly use engagement signals to determine inbox placement. High-quality content drives engagement, which improves deliverability in a virtuous cycle:

Good content → readers open and click → email providers see engagement → future emails land in Primary → more opens and clicks

The reverse is equally true: boring content → low engagement → emails routed to Promotions/Spam → even lower engagement.

Image-to-Text Ratio

Emails that are mostly images with little text look like marketing emails to spam filters. Keep a healthy text-to-image ratio:

  • Don’t send emails that are just one large image
  • Include substantive text content alongside images
  • Limit images to 5-8 per email

Too many links in an email can trigger spam filters. While there’s no exact threshold, emails with 20+ links are more likely to be flagged. Keep links relevant and purposeful.

Engagement-Boosting Tactics

Since engagement signals affect deliverability, actively encourage interaction:

  • Ask questions: invite readers to reply. Replies are the strongest positive signal for inbox placement.
  • Be conversational: personal-feeling emails get more engagement than formal broadcasts.
  • Front-load value: put the most interesting content at the top so readers engage immediately.
  • Consistent scheduling: subscribers who expect your email at a specific time are more likely to open it promptly.

Gmail’s Promotions Tab

Gmail is the most common email provider, and its Promotions tab is the biggest deliverability challenge for newsletters. Gmail sorts emails using machine learning based on:

  • Sender history with that specific user
  • Content patterns (marketing-style formatting triggers Promotions)
  • User behavior (if a user moves your email to Primary, future emails follow)

How to Get to Primary

Ask subscribers to move your email: include a note in your welcome email asking new subscribers to drag your email from Promotions to Primary. This trains Gmail for that specific user.

Encourage replies: Gmail considers emails you reply to as personal correspondence, not promotions.

Simple formatting: heavily styled emails with lots of images, buttons, and HTML look like marketing emails. Substack’s default formatting is relatively simple, which helps.

Consistent sending: sporadic senders are more likely to be sorted into Promotions.

List Hygiene

Why It Matters

A large list of disengaged subscribers hurts deliverability. Email providers track your overall engagement rates. If 60% of your list never opens your emails, that low engagement signal affects delivery for everyone — including your engaged readers.

What Substack Does

Substack automatically handles:

  • Removing hard bounces (invalid email addresses)
  • Managing unsubscribes
  • Suppressing addresses that repeatedly soft bounce

What You Can Do

  • Watch your open rate trends: declining open rates may indicate growing disengagement
  • Re-engage or remove: periodically send a re-engagement email to inactive subscribers asking if they still want to receive your newsletter
  • Focus on quality over quantity: 5,000 engaged subscribers deliver better than 20,000 mostly inactive ones

Whitelisting Instructions

Include instructions in your welcome email for subscribers to whitelist your email address. This varies by email provider:

Gmail: drag the email from Promotions to Primary, or add the sender to contacts

Outlook: right-click → “Never block sender” or add to Safe Senders list

Apple Mail: add the sender’s email to contacts

Yahoo Mail: add the sender’s email to contacts

A brief, friendly instruction in your welcome sequence can significantly improve inbox placement for new subscribers.

Monitoring Deliverability

Substack’s Metrics

Track these metrics in your Substack dashboard:

  • Open rate: a declining trend may indicate deliverability issues
  • Subscriber growth vs. churn: high unsubscribe rates can damage sender reputation
  • Email vs. web views: if web views are proportionally high, subscribers may not be receiving the email version

External Tools

  • Google Postmaster Tools: if you use a custom domain, this shows how Gmail views your sending reputation
  • Mail Tester: send a test email to mail-tester.com for a deliverability score
  • Seed list testing: send to test addresses across different email providers to check placement

Common Deliverability Problems

Sudden Open Rate Drop

If your open rate drops suddenly, possible causes:

  • Email provider changed their filtering algorithm (happens periodically)
  • Your sending IP was temporarily flagged (contact Substack support)
  • A recent email had spam-like characteristics that trained filters
  • You added a large batch of cold subscribers who aren’t engaged

Corporate Email Filters

Subscribers using corporate email (company domains with strict filtering) may have additional barriers. If many of your subscribers are professionals, deliverability to corporate domains is especially important. Custom domain email sending helps here.

Subscriber Complaints

If subscribers mark your email as spam (even once), it’s a strong negative signal. Minimize this by:

  • Making unsubscribe easy and visible
  • Not emailing more frequently than expected
  • Matching content to what subscribers signed up for

Key Takeaways

  • Substack handles core deliverability infrastructure (SPF, DKIM, IP management)
  • Subject lines, content quality, and engagement signals are what you control
  • Gmail’s Promotions tab is the biggest challenge — ask subscribers to move you to Primary
  • Encourage replies: they’re the strongest positive signal for inbox placement
  • Maintain list hygiene: disengaged subscribers hurt deliverability for everyone
  • Keep a healthy text-to-image ratio and reasonable number of links
  • Include whitelisting instructions in your welcome email
  • Monitor open rates for declining trends that might indicate deliverability issues
  • Quality content creates a virtuous cycle: engagement → better placement → more engagement