TL;DR
The best subject lines are 3–5 words, conversational, and sound like something a colleague would send — not a marketing email. Think: 'Quick question, [Name]' or 'Your SDR ramp time' or '[Their company] + [your company].' Avoid clickbait, questions with obvious answers, and anything that looks like a newsletter.
Subject lines have one job: get the email opened. That's it. The subject line doesn't close deals — it just gets the door open.
What works:
*Conversational and short (3–5 words):* - "Quick question, [First Name]" - "Saw your LinkedIn post" - "[Their company] outbound motion" - "SDR ramp time at [Company]?"
*Pattern interrupts that don't look like marketing:* - "[Name]?" (one word, their name — gets opened out of curiosity) - "Referral from [Mutual Contact]" (even better if there's a real connection) - "Re: [topic you know they care about]"
*Context-specific hooks:* - "Series A + demand gen timing" - "Your recent [blog post title] — question" - "Hiring 3 SDRs?"
What doesn't work:
- Subject lines with > 7 words (truncated on mobile, look like marketing) - Questions with obvious yes answers ("Want more leads?") - Fake urgency ("Time-sensitive opportunity") - Anything with "free," "guaranteed," "no risk," or excessive capitals - "[First Name], I can help your company..." - Subject lines that try to summarize your pitch
The personalization play: The most effective subject lines reference something specific to the recipient that shows you actually researched them. "[Company] + SDR ramp time" outperforms "SDR ramp time" because it signals this wasn't a blast.
Testing framework: A/B test two subject lines per campaign, minimum 200 sends per variant, track open rate (not just reply rate). Run three weeks of testing, then commit to the winner for the next month before testing again.
From Cactus: Cactus A/B tests subject lines across every client campaign — our current top-performing format is a 3-word phrase referencing either the prospect's company or a specific trigger event.
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Book a free strategy call →How do I write a cold email that gets replies?
Write one sentence that's specifically about them, one sentence on their problem, one on your solution, and one CTA. The email should be under 80 words, reference something real about their company, and ask a yes-or-no question at the end.
How do I find leads for cold email?
Start with Apollo.io or LinkedIn Sales Navigator to build a list from your ICP criteria. Enrich it through Clay or a waterfall of data providers for verified emails. Target trigger events — recent funding, new hires, job postings — to catch companies in active buying moments.
What is a good cold email reply rate?
A good cold email reply rate is 3–8% for a broad ICP campaign and 8–15% for a highly personalized, trigger-event-based campaign. Anything above 15% with meaningful volume is excellent. Below 2% means something fundamental is wrong — ICP, targeting, or the email itself.
How many cold emails should I send per day?
New domains should warm up for 2–4 weeks before sending real campaigns, starting at 10–20 emails/day and capping out at 30–50/day per domain for sustained campaigns. With multiple warmed domains running in rotation, total volume can reach 500–2,000+/day without domain damage.