How to Use This Swipe File
Subject lines are the single most important factor in whether your email gets opened. This collection of 50+ proven subject lines is organized by approach. Use them as inspiration and customize for your specific audience.
**Important principles:**
- Always test subject lines — what works in one industry may not work in another
- Personalize whenever possible (company name, first name, role)
- Keep subject lines under 7 words for optimal mobile display
- Use lowercase for a more natural, conversational feel
- Never use all caps, excessive punctuation, or emoji in cold outreach
Category 1: Curiosity-Based Subject Lines
Curiosity-based subject lines work by creating an information gap that the recipient wants to close.
- quick question about [Company]
- noticed something about [Company]
- idea for [Company]'s [department/initiative]
- thought about [specific topic]
- [Company]'s approach to [challenge]
- something [similar company] is doing differently
- interesting trend in [their industry]
- what [competitor] is getting wrong
- question about your [process/strategy]
- the [Company] opportunity
When to use: Best for first-touch emails where you need to earn attention. Works well when you have a genuinely interesting insight to share.
When to avoid: If your email body does not deliver on the curiosity, the prospect will feel baited. Only use when you have substance to back it up.
Category 2: Value Proposition Subject Lines
These subject lines lead with the benefit or outcome.
- [X]% more [metric] for [Company]
- scaling [their goal] without [common pain]
- [result] in [timeframe] — here is how
- reducing [pain point] by [percentage/amount]
- how [similar company] solved [problem]
- a better way to handle [process]
- [specific metric] improvement for [their industry]
- [their goal] without adding headcount
- cutting [process] time in half
- the [metric] gap at [Company]
When to use: When you have strong proof points and the prospect is likely already aware of the problem. Works well for re-engagement and later-sequence emails.
When to avoid: If your claims are too bold without backing data, these can feel spammy. Always be prepared to substantiate.
Category 3: Personalization-Based Subject Lines
These use prospect-specific details to demonstrate relevance.
- loved your post on [topic]
- [mutual connection] suggested I reach out
- fellow [shared group/alma mater/trait]
- congrats on [recent achievement]
- following up from [event/conference]
- saw [Company]'s [recent news/launch]
- re: [something they published or said]
- your [recent talk/podcast] on [topic]
- [Name], quick thought on [their initiative]
- re: [Company]'s [specific project or product]
When to use: Always effective, but especially for high-value prospects where you have done real research. The personalization must be genuine and specific.
When to avoid: If the personalization is forced or generic ("love what you guys are doing"), it hurts more than it helps.
Category 4: Social Proof Subject Lines
These leverage credibility from known brands, results, or shared connections.
- how [well-known company] handles [challenge]
- [their competitor] just started doing this
- what [X] [their industry] companies have in common
- [known company]'s [metric] improvement
- joining [list of known brands] in [activity]
- the approach [number] [their industry] leaders use
- why [known company] switched to [your approach]
- [industry report/study] results are in
- [impressive stat] across [number] companies
- what top [their role]s are prioritizing in 2026
When to use: When you have recognizable brand names or impressive aggregate data. Works especially well in competitive industries.
When to avoid: If you are name-dropping brands you have not actually worked with, or if the social proof is not relevant to the prospect's situation.
Category 5: Question-Based Subject Lines
Questions engage the reader's brain differently — they naturally want to answer.
- open to exploring [topic]?
- is [challenge] a priority this quarter?
- who handles [function] at [Company]?
- thoughts on [industry trend]?
- [Name], can I share something?
- should [Company] be doing [activity]?
- how are you handling [challenge]?
- would this help [Company]?
- time for a fresh approach to [activity]?
- is [their current approach] still working?
When to use: Great for opening conversations and for breakup emails. Questions lower the barrier to replying because the prospect can answer with a simple yes or no.
When to avoid: Avoid questions that feel rhetorical or manipulative. The question should be genuine and relevant.
Subject Line Testing Framework
To systematically improve your subject lines, follow this framework:
**Step 1: Establish a baseline**
- Send your current best subject line to a segment of 100+ prospects
- Record the open rate as your baseline
**Step 2: Create variants**
- Write 3-4 alternative subject lines using different categories from this swipe file
- Each variant should test one specific element (curiosity vs. value, question vs. statement, etc.)
**Step 3: Run the test**
- Split your audience evenly across variants
- Send at the same time and day to control for timing effects
- Use a minimum sample size of 50 per variant
**Step 4: Analyze results**
- Compare open rates across variants
- Look for statistically significant differences (at least 5% delta)
- Consider reply rates too — a high open rate with low replies means the subject line set wrong expectations
**Step 5: Iterate**
- Take your winning subject line and test new variants against it
- Repeat every 2-4 weeks as performance changes over time
- Document learnings in a shared file for your team
What to Avoid
These patterns consistently underperform in cold outreach:
- **"Touching base"** or **"Checking in"** — overused and provides no value signal
- "[Company] + [Your Company]" — feels like a sales pitch immediately
- "Introduction" or "Intro" — too generic, easy to ignore
- All caps or excessive punctuation — triggers spam filters and looks unprofessional
- Emoji — polarizing and often flagged by spam filters in cold email
- Long subject lines — anything over 60 characters gets cut off on mobile
- "Re:" or "Fwd:" when it is not a reply — feels deceptive and damages trust
- "Partnership opportunity" or "collaboration" — overused by mass emailers
- Urgency words like "urgent," "time-sensitive," "act now" — spam trigger
Key Takeaways
- Subject lines determine whether your email gets opened — invest time in getting them right
- Use a variety of approaches (curiosity, value, personalization, social proof, questions)
- Always personalize with the company name, prospect name, or specific detail
- Keep subject lines short (under 7 words), lowercase, and natural-sounding
- Test systematically and document what works for your specific audience
- Avoid overused phrases, spam triggers, and deceptive patterns