How to write really good subject lines for cold email?

According to an Invesp study, 47% of email recipients open an email based on the subject line, while 69% mark it as spam just because of it. In other words, the subject line can make or break your email. I've been on both sides – had my fair share of bombs and some gold mines.
I tested a ton of them to figure out what works and what’s a waste of time. So, in this article, I’m dropping the real tips based on my trial and error that’ll get your emails opened.

‍

How to write subject lines that feel internal?

A good cold email subject line doesn't scream “sales pitch.” It makes the recipient curious without pulling any sneaky moves.

It feels like an internal email. And people are more likely to open emails that look like they’re from someone within their company.

Your subject line should also be short and minimalistic, often just one or two words. It should be a bit vague and spark curiosity without feeling deceptive.

At all costs, avoid using salesy language, such as “discount,” “offer,” or “free,” as this can make your email feel pushy or untrustworthy. Keep it simple and intriguing to encourage actions.

‍

Examples of internal-looking subject lines:

• "your SDRs" – Used when offering SDR outsourcing services.
• "PM" – Used for a company providing back-office support to property managers.
• "bounces" – Used by a SaaS company offering SMTP relay services.
• "Grant audit" – Used by an ad agency offering Google Ad Grant audits and services.
• "Re: [Job_Posting_Title]" – Works well when reaching out to companies hiring for specific roles.

‍

Examples of subject lines to avoid:

• "Save time as property manager" – Feels more like a generic marketing email.
• "What's the quality of your SDRs?" – Assumes a concern that may not be there.
• "Too many email bounces?" – Assumes the recipient has an issue with email bounces.
• "FREE Grant Audit" – Feels like a typical marketing pitch.
• "Better SDRs?" – Comes off as condescending and overly direct.

‍

Relevance matters.

An average worker gets about 120 emails a day. While some of us might get a little less or more, we can all agree – our inboxes are flooded. This means your email has to stand out if you want it to get opened.

But, be careful here. High open rate is great, but misleading subject lines backfire.

If someone opens your email expecting one thing and gets something entirely different, they’ll likely ignore or delete it – and you’ve lost credibility. Your subject lines must be honest and relevant.

Let me show you the right and wrong way to use exactly the same subject lines. 👇

‍

Good example: 

Subject: "your SDRs"

Hey Antonio,

I saw you're looking to hire new SDRs. I wanted to throw a different option your way — my agency offers appointment setting services. No hiring required.

Does that sound like something you’d be interested in?

Why it works: The subject line directly connects to the email content. It gives context, making it feel relevant.

‍

Bad example:

Subject: "your SDRs"

Hey Antonio,

My agency offers appointment setting as a service. Is this something that might interest you?

Why it doesn’t work: The recipient has no idea why they got this email, making it more likely to be ignored.

Rule of thumb: Your subject line should always match what’s inside the email. If it feels like a bait-and-switch, rethink it and tweak it.

‍

What about “Quick question”?

Using "Quick Question" with the person’s name, like "Quick question, John" checks all the boxes for a good subject line. It’s personalized, creates a sense of urgency and piques the recipient’s curiosity. 

However, while it has been pretty effective in the past, it became overused at some point. 

As soon as everyone started using it, it lost its impact. Now it’s too recognizable and predictable, so I recommend avoiding it.

There are other options that can spark the same (or even higher) level of interest without falling into the mass usage trap.

For example, subject lines like "your SDRs" or "PM" also generate intrigue but are less conspicuous in a sea of almost identical messages.

‍

small caps vs. Capitalized Subject Lines?

How you format your subject line can change how it’s received. Small caps, like "your SDRs," come across as casual and internal, making them perfect for startups, agencies, or more laid-back industries. 

Capitalizing the words, like "Your SDRs," gives a more professional, polished feel, which works better for industries like legal, finance, or compliance.

Also, if targeting senior executives → stick with Capitalized for professionalism

‍

And what about personalization variables?

Go for it. 

Using variables like first_name, company_name, and other specifics make your message feel way more relevant and catchy (especially when paired with AI-driven customizations).

For example, merging a company name with a personalized touch, like "Peakflow x Fyxer," can make a strong impact. 

Keep in mind that adding a company name can be problematic if the name is too long. Also, be mindful of "normalizing" names – leave out things like LLC, LTD, or excessive capitalization. 

‍Pro tip: Pull a list of all the client’s case studies from their site and create a subject line like “your [company name] case study" – e.g., “your Coca-Cola case study.” This shows you’ve done your homework and makes the email feel more personal and engaging.

‍

What to avoid in subject lines?

Certain elements make subject lines feel like spam:

• Numbers (e.g., "10x your leads") – It just sounds like you're bragging and trying way too hard.
• Unnecessary punctuation (!, ?, …) – Feels gimmicky.
• Special symbols (#, $, %, @) – Very common in spam emails and looks like an error.

‍

A/B testing to find out what works.

Even with these guidelines, subject lines are not one-size-fits-all. Always test 2-3 different subject lines at the same time.

Here’s the playbook I use for testing:

• Keep the email body identical while changing only the subject line.
• Track reply rates instead of open rates (since open tracking isn’t always reliable).
• Test across different decision-makers. A subject line that works for a CEO may not work for a CTO.

‍

Final takeaways

Crafting good subject lines is about subtlety, clarity, and constant iteration. 

And remember – some subject lines might be good for one business, but a total failure for yours. What works for one crowd won’t work for another, so don’t be afraid to mix it up until you find what actually gets attention from your audience.

Stick to these principles, keep testing, and you’ll see your engagement and open rates climb. The more you refine your approach, the better your results will get.

‍