Cold email subject lines
Last updated . Figures are approximate and change with the market, so check current prices before you decide.
A subject line has one job: get a busy person to open an email that is relevant to them. It cannot rescue a bad list or a message that lands in spam, which is why the second half of this guide covers deliverability. We do not quote open rates or reply rates here, because we have not found a reliable independent source for them, and numbers from vendors who sell the tools tend to flatter their own products.
Principles
- Be specific. "Question about your Shopify checkout" beats "Quick question" because it shows you know something about the reader.
- Keep it short. Many people read email on a phone, where long subject lines are cut off. Put the important word first.
- Write like a colleague, not a campaign. Plain sentence case reads as a person. Title Case, emoji and exclamation marks read as marketing.
- Match the email. The subject should be a fair preview of the message. This is also the law, as covered below.
- Use one idea. Do not squeeze an offer, a deadline and a question into the same line.
- Do not use the recipient's name unless it adds meaning. A first name in the subject line is a template habit that readers recognise.
- Do not overpromise. If the email asks a question, the subject can be that question.
24 subject lines by situation
Replace the brackets with real details. If you cannot fill the bracket with something true, do not use the line.
You have a mutual contact or a reason
- [Name] suggested I write
- Intro from [mutual contact]
- Saw your talk at [event]
- Your post about [topic]
You noticed something about their business
- Your [product] page loads slowly on mobile
- Question about your checkout
- [Company] and [specific tool they use]
- Hiring a [role]? Related idea
- Congrats on the [launch or news item]
You are asking a question
- Who handles [responsibility] at [company]?
- Is [problem] on your list this quarter?
- How do you handle [process] today?
- Quick question on [specific topic]
You offer something concrete
- Idea for [company]'s [channel]
- Cutting [cost or task] at [company]
- A [format] for [their industry]
- [Number] suppliers for [product], with quotes
Follow-ups
- Following up on my note about [topic]
- Still relevant, [company]?
- Should I close this out?
- One last note on [topic]
Re-engaging after silence
- Anything changed on [topic]?
- Revisiting [topic] for next quarter
Keep any follow-up honest. A follow-up that says it is a follow-up is fine. A subject that pretends the reader already replied is not, as the next section explains.
What to avoid
- Fake "Re:" and "Fwd:" prefixes. They imply a conversation that never happened. The US CAN-SPAM Act compliance guide from the FTC says not to use deceptive subject lines and not to use false or misleading header information. The same guide states that the law makes no exception for business-to-business email.
- Spam-style wording: "free", "guaranteed", "act now", "100% off", rows of exclamation marks and all capitals. Filters weigh many signals and no single word decides, but these phrases add little and cost you trust.
- Clickbait that the email does not answer.
- A different sender name from the real sender. The FTC guide says "From," "To," and routing information must be accurate and identify who sent the message.
- Subject lines that try to be clever at the cost of clarity.
The legal basics
These rules are a summary, not legal advice.
In the United States, the FTC's CAN-SPAM guide requires accurate sender information and a valid physical postal address in each message. It also requires a clear way to opt out, and says to honour an opt-out request within 10 business days. The FTC says each separate email in violation can bring a penalty of up to $53,088.
In the EU, the ePrivacy Directive governs marketing email. Article 13 allows direct marketing by email only with prior consent, with an exception for a seller's existing customers, and it bans marketing email that disguises the sender's identity or has no valid address to ask for it to stop. Member states apply the rule to company recipients differently, and some are stricter than others. The GDPR also applies when you process the personal data of the people on your list. For that reason we do not send cold email to Spain, Germany or Austria. If you sell into the EU, take advice for each country.
In every case: identify yourself, say why you are writing, and make opting out easy and honoured.
Why cold emails land in spam
A good subject line only matters if the email reaches the inbox. The usual causes are not wording.
Authentication
Mailbox providers want proof that you are allowed to send from your domain. Google's sender guidelines say that since 1 February 2024 all senders to personal Gmail accounts must set up SPF or DKIM, use TLS, keep valid DNS records and keep their reported spam rate below 0.3%. Google recommends staying under 0.10%. For senders of more than 5,000 messages a day, it also requires SPF, DKIM and DMARC, one-click unsubscribe on marketing messages, and alignment between the From domain and the authenticated domain.
Most small cold email programmes send far fewer than 5,000 a day. That does not make authentication optional in practice. SPF, DKIM and DMARC remain the safe baseline for any sender, and our free email check tests part of your own setup.
List quality and complaints
A bounced address damages your sender reputation. So does a recipient who clicks "report spam". Because Google tracks the spam rate, sending to the wrong people hurts the next send too. Verify addresses, target people who plausibly care and remove anyone who opts out at once.
Sending volume and new domains
A brand-new mailbox that sends a large batch on day one looks like a spam operation. New mailboxes need a warm-up period, and volume should stay modest per mailbox. We use about two weeks of warm-up and cap volume per mailbox. Send from a separate domain, so a problem never touches your main business email.
How to test subject lines
- Test one variable at a time, such as a question against a statement.
- Split a list randomly, so each version reaches the same kind of people.
- Judge by replies, not opens. Open tracking is unreliable and tells you less than a reply does.
- Be realistic about volume. A small list cannot prove one subject line beats another. Use tests to pick a direction and read the replies.
- Write down what you tried, and change only the line that lost.
FAQ
What makes a good cold email subject line?
It is short, plain and specific to the recipient, and it fairly describes the email. A line that names their company, a mutual contact or a precise question usually works better than a generic hook.
How long should a subject line be?
Short enough to read fully on a phone, which in practice means a few words with the important one first. There is no single correct length, so test against your own audience.
Is it legal to use "Re:" in a cold email subject line?
Not when there was no earlier conversation. The FTC's CAN-SPAM guidance prohibits deceptive subject lines, and a fake "Re:" implies a reply that never happened.
Why do my cold emails go to spam?
Common reasons are missing authentication (SPF, DKIM, DMARC), a poor list, too much volume from a new mailbox and complaints from recipients. The subject line is rarely the main cause.
Is cold email legal in the EU and UK?
It depends on the country and on whether the recipient is a person or a company. The ePrivacy Directive requires consent for direct marketing by email by default and bans disguised senders. Check the rules for each country before you send.
Should I use emoji or personalisation in the subject line?
Use them rarely. A first name or emoji in the subject is a pattern many readers associate with mass campaigns. Specific content about their business is a better form of personalisation.
How OnePDA can help
Our cold email agency does the parts that decide whether your subject line is ever seen. Setup is €590 and the monthly fee is €690, both excluding VAT where it applies. See also our cold email services.
- A checked list of named decision-makers in one niche and one market
- Separate sending domains and mailboxes, with SPF, DKIM and DMARC configured
- Warm-up of about two weeks, while you approve the copy
- A three-step sequence of short emails, which nothing sends without your approval
- Every reply forwarded within 24 hours
- A report every Monday
Everything stays in your name: domain, accounts and data. It is for selling to businesses in the US, the UK and the EU, and we do not send to Spain, Germany or Austria. We cannot promise orders, meetings or a number of replies, or 100% inbox placement. If cold email is not right for your business, we will tell you.