Best Practices for Writing Email Subject Lines That Don't Get Flagged as Spam

July 14, 2025

Your subject line is the first—and sometimes only—chance to make an impression in your subscriber's inbox. A great subject line can boost open rates, drive engagement, and keep your emails out of the spam folder. In this guide, you'll learn the best practices for writing subject lines that get noticed, get opened, and get results—without triggering spam filters.

Why Subject Lines Matter for Email Deliverability

Subject lines are one of the first things both recipients and spam filters see. A well-crafted subject line can improve your open rates and sender reputation, while a poorly written one can send your emails straight to spam. Understanding what makes a subject line effective—and what gets it flagged—is essential for any marketer.

Common Subject Line Mistakes That Trigger Spam Filters

1. Using Spam Trigger Words

Words like "FREE", "Act Now", "Limited Time", and "Guaranteed" are often associated with spam. Overusing these or similar phrases can get your emails flagged by filters.

2. Excessive Punctuation and ALL CAPS

Subject lines with multiple exclamation marks (!!!), question marks (???), or written in ALL CAPS look suspicious to both users and spam filters.

3. Misleading or Clickbait Tactics

Promising something in the subject line that isn’t delivered in the email, or using deceptive tactics, can lead to spam complaints and damage your sender reputation.

4. Overly Long or Vague Subject Lines

Subject lines that are too long may get cut off, while vague lines fail to communicate value. Both can reduce open rates and increase the risk of being ignored or marked as spam.

Best Practices for Writing Effective Subject Lines

1. Be Clear and Specific

Let recipients know exactly what to expect. Clear, direct subject lines outperform vague or mysterious ones. Example: "Your June Newsletter: 5 New Features Inside".

2. Personalize When Possible

Use the recipient’s name or reference their interests. Personalization increases open rates and makes your emails feel more relevant.

3. Keep It Short (40-60 Characters)

Most inboxes display only the first 40-60 characters. Aim for concise subject lines that communicate value quickly.

4. Avoid Spammy Language

Steer clear of words and phrases commonly flagged by spam filters. Focus on value, not hype.

5. Use A/B Testing

Test different subject lines to see what resonates with your audience. Use data to refine your approach and improve results over time.

6. Create a Sense of Urgency (But Don’t Overdo It)

Phrases like "Last Chance" or "Ends Tonight" can boost opens, but use them sparingly and only when genuine.

7. Use Emojis Thoughtfully

Emojis can help your subject line stand out, but use them in moderation and make sure they’re relevant to your message and audience.

Examples of High-Performing Subject Lines

Welcome, [First Name]! Here’s What’s New This Month

Don’t Miss Out: Your Exclusive Offer Ends Tonight

We Miss You! Here’s a Special Discount Just for You

How to Improve Your Email Open Rates in 2025

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How to Test and Optimize Your Subject Lines

A/B Testing: Send two versions of your email with different subject lines to a small segment of your list. Use the winning subject line for the rest of your audience.

Spam Score Tools: Use tools that analyze your subject lines for spam triggers and provide suggestions for improvement.

Monitor Metrics: Track open rates, click rates, and spam complaints to see how your subject lines are performing over time.

The Future of Subject Lines in Email Marketing

AI-Powered Personalization: Expect more tools that use AI to generate and optimize subject lines based on recipient behavior and preferences.

Increased Focus on Engagement: Email providers are prioritizing engagement metrics, so subject lines that drive real interaction will become even more important.

Compliance and Privacy: As privacy regulations evolve, transparency and honesty in subject lines will be critical for both compliance and deliverability.

Take Action: Start Writing Better Subject Lines Today

Don’t let your emails get lost in the spam folder. Use these best practices to craft subject lines that get noticed, get opened, and get results. Start testing and optimizing your subject lines today to boost your email marketing ROI.

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Remember: Your subject line is your first impression. Make it count by following these best practices and continually testing and improving your approach. Your subscribers—and your results—will thank you.