Your open rate is dropping and nobody knows whether the copy or the inbox is to blame. How do you tell? An email gets read when two things are settled, in this order. It reaches the inbox first. Then its subject line and first two lines tell the recipient what you expect from them.
Email copywriting is the writing of the subject line, the preheader, the body and the call to action. An email copywriter makes a living from it: every word serves one specific action (a click or a reply).
Before the first line: a clean list, reputation, inbox placement
Copy sent to an address that no longer exists produces only a hard bounce, a permanent refusal from the recipient’s server (SMTP code 550 5.1.1: unknown user). A soft bounce is temporary (code 452 4.2.2: mailbox full). A high hard bounce rate points to a neglected list and lowers your domain’s sender reputation. The most common email deliverability errors start there, before anyone debates the quality of the text.
Gmail puts numbers on what it tolerates. Since February 1, 2024, any sender writing to Gmail accounts must keep its spam rate in Postmaster Tools under 0.3%. Google recommends staying under 0.10% and never reaching 0.30% (Google’s email sender guidelines). All senders must authenticate their messages with SPF or DKIM. Above 5,000 messages a day to Gmail, SPF, DKIM and DMARC are all required, though the DMARC policy can stay at p=none.
Copywriting feeds into that rate. A subject line that promises what the body does not deliver pushes readers to hit “Report spam”, and every report adds to your complaint rate. Catch-all domains, which accept any address, hide part of the dead addresses.
To follow the rate, check the spam rate in Postmaster Tools and look at the curve over the last few weeks. A rise after you changed the copy points at the copy. A rise after you imported a new file points at the list.
Before your next campaign, run a sample of your file through CaptainVerify’s list verification. You get the hard bounce rate you would have sent, without sending anything.
Subject line and preheader: what the recipient sees before opening
The subject line decides whether the email is opened. The preheader, the short text shown next to or under the subject, completes it. If you leave it empty, the mail app pulls the first text of the body, often “View in browser”. The preheader deserves the same care as the subject: it carries the information the subject has no room for.
Four subject lines, before and after
| Context | Before | After | What changes |
|---|---|---|---|
| Newsletter | October newsletter | Invoice reminders: the 4-line template | The content is announced |
| Promotion | AMAZING DEAL!!! Don’t miss out | Free shipping until Sunday midnight | A dated deadline replaces the capitals |
| Cold email | Introducing our solution | Question about your client follow-ups | The reader’s topic comes before the sender’s |
| Customer reminder | Reminder | Your quote expires Friday: one reply is enough | The consequence and the action are in the subject |
Capitals and unproven numerical promises push a message toward the filters. The list of words to avoid in an email shows which ones trigger alerts. For formulas that work, see these eye-catching email subject lines.
On the preheader side, here is a before and after: “View in browser” (the default text) becomes “Just reply yes and we’ll lock in the current terms”. The full pair fits on two lines. Subject: “Your quote expires Friday”. Preheader: “Just reply yes and we’ll lock in the current terms.” The preheader answers the question the subject raises without repeating it.
Message length and reading on mobile
Hunter analyzed 34 million cold emails sent by its users between 2022 and 2024. Messages of 20 to 39 words get the best average reply rate, 4.5%. Those under 100 words average 3.9% replies against 3.7% for longer ones (Hunter’s study on cold email length). The study’s author notes himself that effective lengths vary by context.
On the reading side, Litmus measured more than a billion opens in July 2026. Apple Mail (62.26%) and Gmail (27.03%) account for nearly 90% of them. Mobile devices remain the main reading environment for most subscribers (Litmus report on email client market share). So test every send on your phone before launching it. The number of visible subject characters varies from one app to the next: Gmail, Apple Mail and Outlook Mobile do not cut at the same place.
A heavy template slows rendering: the impact of email weight shows up in kilobytes before it becomes a reading problem. For the order of the blocks, the structure of an email stays the same as on a desktop screen: the main information goes in the first two lines.
AIDA and PAS: two outlines for the same message
An outline keeps the text from wandering. AIDA fits when the reader does not yet know they have a need. PAS fits when they do.
AIDA: attention, interest, desire, action
Each letter maps to a block of one or two sentences. Here is an invitation to download an invoice reminder template.
Subject: Invoice reminders: the 4-line template
Could your invoice reminders fit in four lines? (attention)
The template includes the invoice reminder with its amount. It adds the due date and the payment link. (interest)
You copy it, change the amount, send it. (desire)
Download the template in Word format. (action)
PAS: problem, agitation, solution
This outline names a difficulty the reader has, shows the consequence, then offers a way out. It suits cold email because it needs few words.
Subject: Question about your client follow-ups
Hi Claire, are your payment reminders still going out by hand? (problem)
Every week of delay pushes your cash-in back by a week. (agitation)
An automatic reminder on day 3 means you never have to think about it. Want to see the template? (solution)
The same topic written without an outline gives this: “We are an innovative company specializing in financial management solutions and would love to discuss your needs.” That is 17 words, with nothing about what the reader gains and no question to answer.
| Version | Text | Why |
|---|---|---|
| Before | We would love to discuss your needs. | No topic, no action requested |
| After | Still sending payment reminders by hand? An automatic one on day 3 means you never have to think about it. Want the template? | A named problem, a solution, one question |
Cialdini’s six principles applied to an email
Robert Cialdini described six levers of persuasion in Influence. Each one turns into a writing move:
- Reciprocity: give before you ask. A link to a template or a guide comes before the offer of a meeting.
- Commitment and consistency: get a small yes before the big one. “Reply YES to get the template” prepares the next request. A study reported by Influence at Work (Cialdini’s site) cut missed appointments by 18% in health centers by asking patients to write the date on their appointment card themselves (the example comes from healthcare, so check the effect on your own sends).
- Social proof: cite a real subscriber count or a named customer, with their permission. An invented number gets checked fast.
- Authority: sign with your job title and name the source of any figure.
- Liking: use the recipient’s vocabulary and mention a real common point (same industry, same event).
- Scarcity: announce a true deadline, for example “until October 31”. A fake urgency gets the message reported.
Cialdini now lists seven: unity, the feeling of belonging to the same group, joins the first six, according to Influence at Work. Each principle needs dosing: an email that stacks scarcity and social proof by the third line sounds like an ad. Pick one per message and leave the others to the sequence. For the words that trigger these levers, see the emotional trigger words in emailing.
One call to action, phrased as a verb
“Click here” says nothing about what follows the click. “Download the template” or “Book 15 minutes” describe the outcome. Place it once in the body. Two different actions in one email split the response. These call to action tips go through the wording for each goal.
Cold email or newsletter: two contracts with the reader
A newsletter recipient subscribed. A cold email recipient asked for nothing. That difference sets the tone. It also sets the length and what you measure.
| Criterion | Cold email | Newsletter |
|---|---|---|
| Goal | Get a reply | Get a read, then a click |
| Length | Short, under 100 words | Several short blocks, a summary at the top |
| Subject line | A question about the reader’s topic | The content of the issue, named |
| Metric | Reply rate | Clicks and CTOR (click-to-open rate) |
| Main risk | Hard bounces on an old file | Unsubscribes and Promotions tab placement |
For cold email, check the CNIL rules on B2B prospecting that apply to your recipients (CAN-SPAM in the US; in the EU, prior information, a right to object, and legitimate interest only if the message relates to the recipient’s role) before the first send. For a newsletter, an overly sales-heavy message ends up in the Promotions tab. Emails that land in the Promotions tab are mostly sorted there because of heavy templates and sales phrasing.
Email sequences: one message, one job
A sequence is a series of messages triggered by an event or by the absence of a reply. As a rule of thumb, we recommend four messages at most, spaced two to seven days apart. Each message adds something new: a question in the first, a resource in the second. The last one is just a two-line nudge.
Two housekeeping rules save you trouble. Stop the sequence as soon as a reply arrives. After the first send, remove any address that hard bounced so later messages do not hit a mailbox that does not exist. A soft bounce gets one retry, then is set aside if the error persists.
Testing a variant: what to measure since Apple’s privacy changes?
Measure clicks or replies. Opens have been skewed since Apple launched Mail Privacy Protection with iOS 15 in September 2021.
An A/B test compares two versions of a single element (the subject line or the first paragraph) on two randomly drawn halves of the same list. Change only one element at a time.
Measurement is the sticking point since Mail Privacy Protection. Apple’s feature preloads images, which counts as an open, so a large share of opens are no longer real. Comparing two subject lines on open rate then means comparing noise. Measure clicks or replies instead. The email open rate keeps an indicative value inside a single segment, nothing more.
Also plan for enough recipients. With a few dozen people per variant, a gap of two replies is chance. The metrics to track, in order, are covered in the article on marketing KPIs.
Frequently asked questions
What is email copywriting?
It is the writing of an email’s text to get a measurable action: the subject line and preheader, the body and the call to action. It is judged on clicks or sales.
How long should an email be?
A cold email stays under 100 words: in Hunter’s data the gaps between length brackets are small (3.7% to 4.5% replies). A newsletter is split into short blocks. In both cases, the main information goes in the first two lines.
How many recipients do you need for an A/B test?
There is no universal threshold. With a few dozen people per variant, a gap of two replies is chance: aim for several hundred and wait for a clear gap.
How many follow-ups should you send?
Four messages at most is a reasonable benchmark. Stop sending at the first reply and remove hard-bounced addresses.
Rewrite your next send
Open your last email on your phone and reread the subject line and the first two lines. Rewrite them using one of the before/after examples above, and send version B to 10% of the list.
