Subject Lines: Avoid These 3 Common Mistakes That Hurt Sales

Avoid these subject line mistakes to boost open rates and drive sales.

Subject lines determine whether your email gets opened or ignored.

For eCommerce brands relying on email to drive revenue, poor subject lines quietly undermine the entire campaign. Most inboxes are crowded. Mobile users scroll fast. And shoppers decide in seconds whether to tap or delete.

Small improvements in subject lines can lead to higher open rates, better engagement, and stronger sales. At Shawfire Media, we’ve reviewed hundreds of campaigns and consistently see the same three mistakes. Here’s how to fix them.

Mistake 1: Using Language That Triggers Spam Filters

Subject lines filled with promotional buzzwords or exaggerated language are often flagged by inbox providers. Words like “free money,” “guaranteed,” “exclusive,” or phrases written in all caps tend to lower deliverability. They may also make your email sound automated or pushy.

Shoppers are used to filtering through promotional noise. Even if your message makes it to the inbox, this type of language blends in with low-quality offers and gets skipped.

What to do instead:

Use clear, natural phrasing that sounds human.
Write like you’re speaking to one person, not a crowd.
Focus on what matters to the reader, not hype.

Example:
Replace: “🔥 FREE GIFT INSIDE – ACT FAST!”
With: “Something new just landed for you…”

According to Mailmodo, over 70% of consumers say they’re less likely to open emails that feel overly promotional.

Mistake 2: Subject Lines That Are Too Long for Mobile

Most emails are read on phones. If your subject line runs too long, it will get cut off, making the message incomplete or unclear. The optimal length for subject lines is under 42 characters. Anything longer risks being shortened on smaller screens.

Long subject lines often bury the most important words. Readers may miss the point or skip the email altogether.

What to do instead:

Lead with the strongest idea or benefit.
Cut filler words and avoid vague intros like “Introducing” or “Just in.”
Test 3 – 5 word subject lines. They often perform better.

Example:
Replace: “Check Out Our Brand New Collection for Summer 2025”
With: “New arrivals. Summer ready.”

Mistake 3: Ignoring the Preview Text

Preview text is the small gray line of text that appears next to the subject line in most inboxes. It acts as a second chance to get someone to open the email. Many brands leave this blank or allow auto-filled content like “Having trouble viewing this email?”

This is a missed opportunity. The preview text should work alongside your subject line to give more context or tease what’s inside.

What to do instead:

Write a unique preview line for every campaign.
Use it to build curiosity or explain what the email offers.
Avoid repeating the subject line or using placeholder text.

Example:
Subject Line: “The cleanser your skin’s been missing”
Preview Text: “Hydrates, refreshes, and now in travel size”

Real-World Example

One of our skincare clients struggled with open rates stuck at 36.9%. Their subject lines were too long, overly promotional, and had no custom preview text. After adjusting the tone, shortening the length, and pairing each subject line with relevant preview text, open rates increased to 52.4%.

The emails didn’t just get seen. They generated more clicks and sales with no change to the product or offer.

Key Takeaways for Subject Lines

Avoid sales-heavy or spam-triggering language
Keep subject lines under 42 characters for mobile
Write custom preview text to support the subject line

Strong subject lines improve visibility, engagement, and revenue. They are one of the simplest areas to optimize for better results.

Shawfire Media helps eCommerce brands get more from their list with clear, tested strategies that go beyond guesswork. If your emails aren’t pulling their weight, start by fixing your subject lines.

Stop losing opens to weak subject lines.

Let Shawfire Media optimize your subject lines and turn your emails into revenue-driving assets.