Email Subject Lines That Boost Open Rates
The subject line decides whether your email gets opened or ignored — and the wrong words can trip spam filters. Here's how to write ones that work.
TL;DR
Write clear, specific, honest subject lines under ~50 characters. Avoid spam-trigger words and excessive punctuation that hurt inbox placement. Test relentlessly.
What makes a subject line work
- Clarity over cleverness — say what's inside.
- Specificity — numbers and concrete benefits beat vague hype.
- Brevity — aim under 50 characters so it isn't truncated on mobile.
- Honesty — clickbait that misleads spikes complaints.
Spam-trigger words to avoid
Words like "FREE!!!", "ACT NOW", "100% guaranteed", "make money fast" — especially in caps with exclamation marks — raise spam scores. They contribute to the content signals covered in how to avoid the spam folder.
Formatting pitfalls
- ALL CAPS reads as shouting (and spammy).
- Excessive punctuation (!!!, $$$).
- Too many emojis.
- Misleading "Re:" or "Fwd:" prefixes.
The subject line is part of deliverability
Subject lines that mislead generate complaints; complaints hurt reputation; reputation decides placement. It's all connected — see deliverability.
Test everything
A/B test subject lines on a sample of each segment, then send the winner. Small lifts compound across campaigns.
Great subject lines deserve great deliverability — send with AutoPMTA. View plans.
Key takeaways
- Be clear, specific, brief (<50 chars) and honest.
- Avoid spam-trigger words, caps and excess punctuation.
- Misleading subjects cause complaints that hurt reputation.
- A/B test per segment and send the winner.
Frequently asked questions
How long should an email subject line be?+
Aim for under about 50 characters so it isn't truncated on mobile devices. Short, specific subject lines tend to perform best.
What words trigger spam filters in subject lines?+
Hype phrases like 'FREE!!!', 'act now', '100% guaranteed' or 'make money fast' — especially in all caps with excessive punctuation — raise spam scores and hurt deliverability.
Do subject lines affect deliverability?+
Indirectly but significantly. Misleading or spammy subject lines generate complaints, which damage sender reputation and push future mail toward the spam folder.