You spend hours setting up your mail server, configure every setting carefully, hit send — and your message lands straight in the spam folder. It’s frustrating, especially when you know your server isn’t sending spam. The truth is, modern email providers like Gmail, Outlook, and Yahoo use hundreds of signals to decide whether a message deserves the inbox.

In this guide, we’ll walk through the most common reasons emails go to spam, and more importantly, how to fix them so your messages reach the inbox every time.

1. Missing or Misconfigured SPF, DKIM, and DMARC

These three DNS records are your email’s digital ID badges.

  • SPF (Sender Policy Framework) tells the world which servers can send mail for your domain.

  • DKIM (DomainKeys Identified Mail) adds a cryptographic signature proving the message wasn’t altered.

  • DMARC ensures SPF and DKIM are aligned and gives you feedback when authentication fails.

If any of these are missing or broken, major email providers assume your messages are spoofed.

To check whether your domain is properly authenticated, run a test with MailTested.com — it will instantly verify your SPF, DKIM, and DMARC records and show where the problem lies.

Mailtested.com email deliverability service

2. Poor IP or Domain Reputation

Even if your configuration is perfect, sending from a new or blacklisted IP can bury you in spam folders. Providers track the reputation of IPs and domains based on previous mail volume, bounce rates, and complaint ratios.

Quick fixes:

  • Use a dedicated IP for transactional mail.

  • Warm up new IPs by sending small volumes first.

  • Check if your IP or domain appears on blacklists using MailTested’s IP reputation tool.

  • Avoid sending bulk messages from the same domain used for personal communication.

3. Content That Triggers Spam Filters

Your server might be fine — your email content might not. Spam filters scan messages for certain red flags:

  • Excessive use of “FREE,” “URGENT,” or “LIMITED OFFER.”

  • Overuse of capital letters or exclamation marks.

  • Large images with little text.

  • Hidden links or mismatched URLs.

How to fix it:

Keep the tone natural, personalize your message, and maintain a healthy text-to-image ratio (at least 60% text). Before sending, paste your email into a spam-score checker such as MailTested to see what might trigger filters.

4. Missing Reverse DNS and HELO Configuration

If your server’s reverse DNS (PTR record) doesn’t match the domain you’re sending from, many providers instantly distrust your mail. Likewise, an incorrect HELO hostname can look suspicious.

Run a simple test:

dig -x your.server.ip

Ensure the PTR record returns your sending domain, and that the hostname in your mail server matches it.

 

5. No TLS or Outdated Encryption

Email providers now expect encrypted transmission via TLS. Servers that still send mail in plain text are considered insecure.

Enable TLS in your SMTP configuration (smtpd_tls_security_level = may in Postfix, for example) and install a valid SSL certificate for your mail host.

6. Low Engagement and Poor Sending Practices

Even if your server is technically perfect, Gmail and Outlook track engagement metrics: opens, replies, deletions, and spam complaints. If most users ignore or delete your emails, your domain’s reputation drops.

Best practices:

  • Always use confirmed opt-in lists.

  • Avoid purchased email databases.

  • Include a clear unsubscribe link.

  • Keep subject lines honest — no clickbait.

7. Final Check — Test Before You Send

The easiest way to avoid spam issues is to test every campaign before sending it to clients.

Use MailTested.comAttachment.tiff to send a sample message and review the detailed report. It checks SPF, DKIM, DMARC, blacklist status, spam score, and even header structure — so you know exactly what to fix before hitting send.

Conclusion

Getting emails into the inbox isn’t luck — it’s deliverability engineering. Once your DNS records, server reputation, and content quality are under control, your emails will start reaching their audience reliably.

Keep monitoring your results regularly, and use deliverability tools like MailTested to stay one step ahead of spam filters.