You have rewritten the first email four times. It is good. Now there is one empty field left at the top, and every subject line you try sounds either like a newsletter or like a stranger asking for a favour.
Most advice on cold email subject lines is a list of a hundred templates. A list helps less than you would think, because the subject line that works is the one that only fits this email to this person. So this guide gives you the rules first, then examples by situation, then the tricks to stay away from.
The short version
Write the email first and the subject line last. Keep it to a few plain words, specific to the person, and honest about what is inside. Write in lowercase or sentence case, the way you would write to a colleague. Then judge it by how many people reply, not how many open.
On follow-ups, do not write a new subject at all. Reply in the same thread and keep the original.
What a subject line is for
People argue about this, and both sides have a point. A subject line cannot rescue a weak email. If the person opens it and the body is generic, they delete it, and the subject did nothing except delay the deletion.
A bad subject line can sink a good email, though. A cold email arrives in an inbox full of colleagues, customers and tools. The reader looks at three things: who it is from, the subject, and the first line of preview text. If those three look like marketing, the email never gets a chance.
So treat the subject line as a filter, not a hook. Its job is to tell the right person that this email is worth ten seconds, and to tell everyone else not to bother. A subject that gets the wrong person to open is a cost, not a win: they read it, feel tricked, and are more likely to mark it as spam.
What a good cold subject line looks like
A good cold email subject line passes four checks:
- It is short. A few words. Long subjects get cut off on a phone, and short ones look like they came from a person.
- It is specific to them. Something that only makes sense for this company or this person: their hiring, their new office, a name you share.
- It is honest. A reader who opens it finds what the subject promised.
- It sounds like a colleague. No title case, no exclamation marks, no emoji, no “Exclusive offer”.
Write it after the email. The subject is a summary of the email’s reason for existing. If you cannot write that summary in a few plain words, the problem is the email, and a clever subject will not fix it.
Use the preview text too. The first sentence of your email shows next to the subject in most inboxes. Do not waste it on “Hope you are well”. Make it the sentence that explains why you are writing, so the subject and the preview read as one thought.
Examples by situation
These are patterns, not lines to copy. Replace the specifics with something true about your prospect. If you cannot, that is a sign you do not know enough about them to write yet.
| Situation | Subject line | Why it works |
|---|---|---|
| They are hiring for the role your product supports | your two SDR openings | Shows you looked at their company, and names a problem they already have |
| Someone referred you | intro from Priya Shah | A name they know is the strongest reason to open. Only use it with permission |
| They recently expanded or launched something | question about the Leeds office | Specific, and a question invites an answer |
| You read something they wrote | your post on onboarding | Shows the email was written for them, not for a list |
| Agency writing to a brand | idea for the spring range | Offers something before asking for anything |
| Same industry, a peer they would recognise | how Northbridge handles renewals | A named peer makes the email concrete. Only use a real example you can talk about |
| You have nothing specific | (do not send yet) | Research one more thing first. A generic subject goes on a generic email |
Notice what is missing from the table. None of these uses the person’s first name as the main idea. “Quick question, Sarah” was personal once. It is now so common that it reads as a merge tag, which is exactly what it is. Put a first name in the email’s greeting, and put something they would recognise in the subject.
Follow-ups keep the original subject
The most common subject line mistake is not on the first email. It is on the follow-ups.
Send each follow-up as a reply in the original thread. The subject becomes “Re:” plus your first subject, and the prospect sees your first email directly underneath. That context is why a follow-up can be two sentences long. Our guide to cold email follow up covers the timing and what each one should add.
A new subject on every follow-up turns one conversation into four separate cold emails from a stranger. Each one has to earn the open again, with less context than the first.
Only a real reply should start with “Re:”. Google’s email sender guidelines say not to send messages with subject lines starting with Re: or Fwd: unless the messages are actual replies or forwards. A first email dressed up as a reply might get one extra open, and it tells the reader in the first second that you are willing to trick them.
How this works in Doreach
In Doreach, a cold email sequence is a campaign with an Email step for the first message and another Email step for each follow-up. The first Email step has a Subject field above the body. It accepts the same merge tags and spintax as the message, so question about {{location|your office}} fills in each person’s saved location and falls back to “your office” when there is none. Always add the fallback. A tag with no value and no fallback renders as nothing, and a subject with a hole in it looks broken. Merge tags and spintax has the current list of tags and how fallbacks work.
On the second and later Email steps, there is a switch called Send as a reply to the previous email. Turn it on and the Subject field disappears. The follow-up reuses the subject of the earlier email, adds “Re:” in front, and lands in the same thread. That way you cannot accidentally write a new subject on a follow-up. Sequence steps explained covers the steps and delays in detail.
Doreach also leaves open tracking off by default. On the campaign’s Schedule tab, Email format defaults to HTML with Plain text (recommended) as an option, and opens can only be tracked in HTML emails. We recommend plain text and judging subject lines by replies, for the reason in the next section. Replies from every mailbox land in one unified inbox, and with stop on reply on, which is the default, the person’s remaining follow-ups stop.
If you send from several mailboxes, use the same subject across all of them. Multiple sender accounts spreads the campaign over your connected mailboxes, which is what most cold email tools call inbox rotation, and the subject test only means something when the subject is the one thing that changes.
Test subject lines by replies, not opens
Open rate used to be the obvious way to compare subject lines. It is now unreliable. Opens are counted through a tiny image in the email, and Apple’s Mail app has a setting, Protect Mail Activity, that downloads remote content in the background when a message arrives rather than when you read it. For anyone using it, an email can count as opened when nobody looked at it.
Reply rate does not have that problem. A reply means a person read the subject, opened the email and decided to answer. Count positive replies separately from “not interested” if you can, but even total replies tell you more than opens.
Run the test cleanly:
- Change one thing. Same list, same email body, same send window. Only the subject differs.
- Split the list at random. Not by company size or by who you added first.
- Wait for enough replies to mean something. Two replies against one is noise. Keep going until the gap is clear, or call it a draw.
- Keep the winner and test it against a new idea. Do not test ten subjects at once on a small list.
If the subjects are close, stop testing subjects. The bigger gains are usually in who you are emailing and what the first two lines say.
What we would not do
- Fake a reply or a forward. “Re: our call” on a first email breaks Gmail’s guidelines and the reader’s trust in one move.
- Write a subject the email does not deliver. The US CAN-SPAM Act says the subject line must accurately reflect the content of the message. Clickbait is a legal risk as well as a bad first impression.
- Use false urgency. “Last chance” or “Time-sensitive” on a cold email from a stranger is not believable, and it reads like spam.
- Shout. Capital letters, exclamation marks and emoji make a cold email look like a promotion.
- Write a new subject for every follow-up. Keep the thread. The context is the point.
- Copy a template word for word. If a hundred other senders use the same line this week, your prospect has already seen it.
What to do next
Pull up your last cold campaign and read the subject lines next to the first sentence of each email. For each one, ask: would this make sense if I sent it to a different company? If the answer is yes, it is not specific enough. Rewrite it around one thing that is only true for that prospect.
Then set up the next campaign so follow-ups reply in the same thread, and pick reply rate as the number you will judge it by. If you want the sending side handled, Doreach sequences send the first email and its follow-ups from your own mailbox, keep the thread, and stop when someone answers. What will your next subject line say that only one company could receive?
