Subject line tester
Scores your cold email subject line, shows it in Gmail and Outlook on desktop and phone, and suggests up to five better ones.
Ready to send
80 and up is ready to send3 rules cost points. Start with generic subject: "Quick question" could go to any company. It reads as outreach, not a note about their work.
How it shows in the inbox
With your first line as the preview text.What cost points
Biggest first. Each rule comes from the subject line rules Ken writes with.Better lines to test
Ken AINext
What the subject line tester checks
Type a subject line, and the email's first line if you have it. The tester scores the subject from 0 to 100 and lists the reason for every point lost:
- Length in words and characters, and where a phone cuts it off.
- Spam words from any tier, ALL CAPS, exclamation marks, emoji, and $ or % signs.
- A fake "Re:" or "Fwd:" on a first email.
- Placeholders nobody filled in, such as [Company Name] or a {{ that never closes. A normal {{company}} merge field is fine.
- Numbers, questions, and anything that reads like a newsletter or a promotion.
It also shows how the line looks in a Gmail and an Outlook inbox list, on desktop and on mobile, next to your first line.
Cold email subject lines are not marketing subject lines
Most subject line testers score for newsletters: urgency, emoji, curiosity gaps, a number up front. In cold email those habits make the message look like bulk mail from a stranger, and it gets deleted.
The cold subjects that get opened look like internal notes. Ken's default rules for writing them:
- Fewer than 6 words, ideally 3 or 4.
- No emoji, no punctuation, no questions.
- Lowercase except the first letter and names.
- Built on one fact about the prospect, the same fact the first line uses.
- Never "quick question", "introduction", "partnership", "collaboration" or your product name.
The final test: if the subject could go to any other company unchanged, it isn't done.
How to read your score and use the alternatives
Points come off for each rule the line breaks, and the reasons are listed by cost. A never-tier spam word, a fake "Re:", an unfilled placeholder and a very long subject cost the most, because they can hurt delivery as well as opens. Form, such as punctuation or Title Case, costs less.
The AI pass judges clarity, curiosity without clickbait, and whether the subject matches the first line, then writes up to 5 alternatives with the same intent. Pick one, put your real prospect fact into it, and test it again.
Then test it on real sends. A tester can rule out bad lines. Only replies tell you which good line wins.
Why the subject line matters for deliverability
Filters read the subject too. SpamAssassin, the rule engine behind many corporate gateways, has rules that look only at the subject line, such as an ALL CAPS check. A clean subject keeps your spam budget for the body, where one specific number actually helps.
A deceptive subject line is also a legal problem. The US CAN-SPAM Act bans subject lines that mislead the recipient about what the message contains, and a fake "Re:" implying a conversation that never happened can count as one.