Email Deliverability 101: Why Your Campaigns Land in Spam

You can write the perfect subject line, have a clean list, and send an email with genuinely useful content — and it can still end up in spam. Email deliverability is a separate problem from email content quality, and businesses that do not understand the distinction often conclude that email marketing does not work, when the actual problem is that their emails are not reaching the inbox in the first place.

Why emails go to spam

Email providers — Gmail, Outlook, Yahoo — use a combination of technical checks and engagement signals to decide whether an incoming email should land in the inbox, the promotions tab, or the spam folder. Getting filtered to spam does not mean you have done something obviously wrong; it often means that a combination of technical setup issues and sending pattern signals have pushed your score below the threshold for inbox placement.

The technical authentication records — non-negotiable

Three DNS records work together to tell email providers that your messages are legitimate. If these are not set up correctly, your emails are far more likely to be filtered regardless of their content.

SPF (Sender Policy Framework) specifies which servers are authorised to send email from your domain. A missing or misconfigured SPF record means receiving servers cannot verify that your email actually came from you, which is a significant spam signal.

DKIM (DomainKeys Identified Mail) adds a cryptographic signature to every email that the receiving server can verify — essentially a digital stamp that proves the message has not been tampered with in transit and genuinely comes from your domain.

DMARC (Domain-based Message Authentication, Reporting and Conformance) tells receiving servers what to do with emails that fail SPF or DKIM checks — whether to deliver them anyway, quarantine them to spam, or reject them entirely. It also gives you reports on who is sending email from your domain, which is useful for spotting if your domain is being spoofed in phishing attacks.

Your email marketing platform (Mailchimp, Zoho Campaigns, Brevo, or whichever you use) should walk you through setting these up for your sending domain. If they do not, or if you are not sure whether they are configured, ask the platform's support team — this is a standard setup step that any competent provider can help with.

Engagement signals that affect inbox placement

Beyond technical authentication, email providers look at how recipients interact with your emails. High open rates and click rates are positive signals. High spam complaint rates (people clicking "report spam") are severely negative. High bounce rates — sending to invalid email addresses — are also a major red flag that can get your sending domain or IP blacklisted.

This is why list hygiene matters. Remove bounced addresses immediately after a campaign, and periodically remove subscribers who have not opened any of your last 10–15 emails. A smaller, engaged list delivers better than a large, disengaged one — in both business outcomes and deliverability terms.

The sending volume problem

Suddenly sending a large volume of email from a domain that has not sent much email before is a common trigger for spam filtering. Email providers treat it as suspicious behaviour — a pattern associated with compromised accounts and spam operations. If you are starting a new email marketing programme or switching to a new sending domain, warm up the sending volume gradually over a few weeks rather than immediately sending to your full list.

How to test your deliverability

Tools like Mail-Tester.com (free) and GlockApps allow you to send a test email and receive a detailed report on your authentication records, spam score, and predicted inbox placement across major email providers. Running one of these checks before launching a campaign is a five-minute step that can catch problems before they affect thousands of emails.