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Deliverability

Spam Filter

Software that analyzes incoming emails and blocks or flags suspected spam.

Definition

A spam filter is a program that examines incoming email messages and uses various criteria to identify and block unwanted or malicious messages. Filters analyze sender reputation, authentication results, content characteristics, and recipient behavior to determine whether to deliver an email to the inbox, spam folder, or block it entirely. Modern spam filters use machine learning and constantly evolve based on user feedback.

Why It Matters

Understanding how spam filters work helps you craft emails that reach the inbox. Spam filters can block legitimate marketing emails if they trigger certain signals. Knowing what filters look for allows you to optimize your emails for deliverability. Getting past spam filters is essential for any email marketing success.

How It Works

Spam filters use multiple layers of analysis. They check the sender's reputation score and authentication records (SPF, DKIM, DMARC), scan content for spam patterns (certain words, suspicious links, image-to-text ratio), analyze email structure (hidden text, misleading links), and consider recipient behavior (do people mark similar emails as spam?). Each signal contributes to an overall spam score.

Example

Consider two emails:

Email A has "FREE MONEY!!! ACT NOW!!!" as the subject, comes from an unauthenticated domain, is 100% images with no text, and has links to suspicious domains. The spam filter gives it a score of 9/10 (highly likely spam) and blocks it.

Email B has "Your March Newsletter" as the subject, comes from a domain with proper SPF/DKIM/DMARC, has a good text-to-image ratio, includes an unsubscribe link, and the sender has good reputation. Spam score: 1/10. It lands in the inbox.

Best Practices

  • 1Maintain proper authentication (SPF, DKIM, DMARC with p=reject)
  • 2Avoid spam trigger words and excessive punctuation in subject lines
  • 3Balance text and images (aim for 60/40 text to image ratio)
  • 4Include a clear, easy-to-find unsubscribe link in every email
  • 5Send relevant content to engaged subscribers who expect your emails
  • 6Test emails with spam checkers before sending

Frequently Asked Questions

Common triggers include: ALL CAPS text, excessive punctuation (!!!), spam trigger words (FREE, WINNER, URGENT, ACT NOW), poor sender reputation, missing or failing authentication, image-only emails with no text, hidden or tiny white text, misleading subject lines, and suspicious or shortened links.

Use spam testing tools like Mail-Tester.com, GlockApps, or Litmus. Send your email to their test address and get a detailed score with specific suggestions for improvement. Most ESPs also have built-in spam checkers. Aim for a score above 8/10 on Mail-Tester.

Even good emails can land in spam due to: sender reputation issues from previous campaigns, the receiving server's aggressive policies, recipient behavior (if many coworkers mark similar emails as spam), being on a blocklist, or sending to a new audience that does not engage. Check Google Postmaster Tools for Gmail-specific issues.