Free AI Subject Line Analyser

Why subject lines matter for open rates

Your subject line is the first (and sometimes the only) thing a prospect reads before deciding whether to open or delete your email. Research consistently shows that 47% of recipients open emails based on the subject line alone, and 69% report email as spam based on the subject line before they've even read the body.

For cold email outreach, where you're reaching people who have no prior relationship with you, a compelling and spam-filter-safe subject line is not optional. It's the difference between a reply and a spam complaint.

What is deliverability risk?

Deliverability risk refers to the chance that your email never reaches the primary inbox. Spam filters at providers like Gmail and Outlook analyse subject lines for trigger phrases, excessive punctuation, all-caps words, and deceptive patterns. A high-risk subject line can route your email to the spam or promotions folder even when your domain authentication is perfect.

Keeping subject lines short, relevant, and free of hype-language is the easiest single change you can make to improve inbox placement.

What drives email engagement?

Beyond avoiding spam filters, a strong subject line gives the reader a reason to open. The highest-performing cold email subject lines share a few traits: they feel personal (using the recipient's name or company), they're specific (a number or concrete detail), and they hint at value without over-promising.

Questions also work well. They invite curiosity and suggest the body will answer something relevant to the reader. The goal is a subject line that feels like it was written for one person, not broadcast to thousands.

Common subject line mistakes to avoid

• Spam trigger words: "FREE", "Act now", "Guaranteed", "Limited time", "Click here". Filter these out immediately.
• ALL CAPS: Shouting at your recipient is both aggressive and a reliable spam signal.
• Multiple exclamation marks: One is borderline, two or more is a red flag for both humans and filters.
• Too long: Anything over 70 characters will be cut off on most mobile inboxes.
• Too vague: "Quick question" or "Following up" used to work, but now they're so common they're nearly invisible.
• Misleading: Subject lines that don't match the email body damage trust and generate spam complaints.

Tips for writing high-performing subject lines

• Keep it under 60 characters (ideally 35-50).
• Use the recipient's first name or company name: "{{first_name}}, quick thought on your outreach".
• Include a specific number: "3 ideas for your Q2 pipeline".
• Ask a relevant question: "Still struggling with cold reply rates?"
• Avoid hype. Write like a human, not an ad.
• A/B test at least two variations before committing to a sequence.