Email deliverability is a reputation problem as much as a technical one. Internet service providers maintain dynamic reputation scores for every sending domain. These scores reflect recipient behavior: how often your emails are opened, how often they are marked as spam, and how often they bounce. A positive reputation means your emails land in the inbox. A damaged reputation means they go to spam or are blocked entirely.
The technical foundation of good deliverability is three DNS records: SPF, DKIM, and DMARC. SPF specifies which servers are authorized to send on behalf of your domain. DKIM adds a cryptographic signature to your outgoing emails. DMARC tells receiving servers what to do with emails that fail authentication checks. Most email providers walk you through adding these records when you set up your account. They are DNS changes that take minutes to implement.
List quality is the behavioral complement to technical setup. Sending emails to addresses that bounce, that belong to people who never open your emails, or that have marked your emails as spam damages your sender reputation with every send. Clean your list regularly: remove addresses that have bounced, segment out subscribers who have not opened in ninety days, and honor unsubscribe requests promptly. A small, engaged list delivers reliably. A large, disengaged list erodes reputation.
Subject lines that trigger spam filters are a common and avoidable problem. Phrases like 'act now,' 'limited time,' and 'free money,' along with excessive capital letters and exclamation marks, trigger filters trained on millions of messages recipients have marked as spam. Writing subject lines that accurately describe your email's content, without artificial urgency, is both better practice and better deliverability strategy.
Testing your deliverability is possible and worth doing before a major campaign. Mail-tester.com generates a test email address you can send to and provides a deliverability score with specific issues identified. Google Postmaster Tools provides ongoing domain reputation data for emails sent to Gmail addresses. These tools surface problems before they become campaigns that land in spam.