If you run cold email campaigns, marketing automation, or transactional emails and your open rates are consistently low, the problem is likely deliverability — not your subject lines. Your emails are landing in junk folders, and your recipients never see them.
Key Terms
- Inbox providers: Gmail, Outlook, Yahoo — the platforms your recipients use to read email.
- Email service providers (ESPs): The software you use to send campaigns — Mailchimp, SendGrid, ActiveCampaign, etc.
- Email filters: Algorithms that decide whether an email reaches the inbox, the promotions tab, or the junk folder.
Why Emails Land in Junk
Two things send your email to junk: the recipient manually marks it as spam, or the inbox provider’s filters classify it as spam automatically. This article focuses on the second — the factors you can control on the technical and content side.
Authentication: The Foundation
Inbox providers verify that you are authorised to send email from your domain. Fail this check and your emails go to junk regardless of content quality.
SPF
A DNS TXT record that lists the servers authorised to send email from your domain. If your ESP is not in your SPF record, inbox providers treat your emails as potentially spoofed.
DKIM
A cryptographic signature added to each email by your sending server. The receiving server verifies this signature against a public key in your DNS. If the signature does not match, the email is flagged as tampered or unauthorised.
DMARC
A policy that tells inbox providers what to do when SPF or DKIM checks fail — quarantine, reject, or allow. Without DMARC, you have no visibility into who is sending email on your domain’s behalf and no control over what happens when authentication fails.
Action item: Set up SPF, DKIM, and DMARC for every service that sends email from your domain — your inbox provider, your email marketing platform, your CRM, your transactional email service. This is not optional. Without it, even legitimate emails will hit junk folders.
Sender Reputation
Inbox providers maintain reputation scores for sender addresses, domains, and IP addresses. Blacklist directories track senders with a history of spam, phishing, or spoofing. If your domain or ESP’s IP appears on a blacklist, deliverability drops sharply.
What to do:
- Check domain reputation at mxtoolbox.com/blacklists.aspx before launching campaigns.
- When purchasing a new domain, check its history. Previously blacklisted domains carry that reputation forward.
- Monitor sender reputation in your ESP’s dashboard. Most platforms flag reputation drops before they become critical.
Content and Structure
Machine learning filters evaluate email content for spam signals. Even authenticated emails with good sender reputation can hit junk if the content triggers filters.
- Avoid spam-trigger language — “free,” “guaranteed,” “act now,” “limited time” — especially in subject lines.
- Ensure subject lines accurately reflect the email body. Misleading subjects are a spam signal.
- Include your business’s physical address and an unsubscribe link. Both are legally required in most jurisdictions and expected by filters.
- Do not use URL shorteners. Shortened links are associated with phishing. Use full, clean URLs.
- Avoid sending attachments in bulk email. Host files on Google Drive or your website and link to them instead.
List Hygiene
Sending to invalid addresses, inactive recipients, or purchased lists damages your sender reputation rapidly. Clean your list regularly. Remove hard bounces immediately. Segment inactive subscribers and re-engage or remove them. A smaller, engaged list outperforms a large, disengaged one in every metric that matters.
Deliverability is infrastructure, not creative. Get the technical foundation right — authentication, reputation, list hygiene — and your email campaigns reach the inbox. Skip it and no amount of subject line testing will fix your open rates.