What a subject line actually has to do
A subject line is the only part of an email a reader sees before deciding whether to open it, so its job is narrower than most people treat it: it does not need to summarise the email, it needs to earn the open. "Newsletter: March updates" describes the email accurately and gives the reader no reason to act on it. "The pricing change we mentioned in February starts Monday" tells the reader something specific has happened and that it affects them, which is why they open it. The rule that follows is simple: a subject line should name a specific thing the reader recognises.
Length matters less than specificity, but it still matters, because most subject lines are read on a phone screen that cuts them off after roughly 40 characters. You can run a subject line through the character counter to see where it gets cut and whether the important word survives the cut.
Preview text is the second half of the sentence
Preview text is the line of grey text an inbox shows next to or below the subject line, pulled by default from the first words of the email unless you write it deliberately. Left to default, it often repeats "Hi," or "View this email in your browser," wasting the one piece of space that could have finished the thought the subject line started. Write the preview text as a continuation: if the subject line asks a question, the preview text answers half of it; if the subject line names a change, the preview text names the date. Treat the two as one sentence split across two fields.
Open with the reader's situation
The first line a reader sees inside the email carries the weight the subject line and preview text built, so it needs to continue talking about the reader. "We're excited to announce our new pricing structure" opens with the sender's feeling about the sender's decision. "Your monthly plan changes to the new pricing on the 1st" opens with the fact that affects the reader, in the reader's terms. The habit worth building is checking the first line of any draft and asking whether its subject is "we" or "you": if it is "we," move that sentence down and start with the fact that matters to the person reading.
Structure the body around one idea
An email that tries to cover three updates, two reminders and a soft pitch usually achieves none of them, because a reader skimming on a phone will act on the first thing that seems relevant and ignore the rest. Give each email one job: one change to explain, one offer to make, one question to answer. If a second idea genuinely needs sending, it earns its own email. Body copy under this discipline tends to run short, three or four short paragraphs at most, because there is only one thing left to say once the extra ideas are removed.
Give the email one call to action
A call to action (CTA) is the single instruction that tells the reader what to do once they have read the email, and an email with three of them, "read more," "shop now," "reply to this email," gives the reader three reasons to do nothing. Decide before drafting what the one action is, then build the whole email toward it: the subject line raises the topic, the body makes the case, and the CTA names the action in words that match what actually happens next. "See the new pricing" tells the reader what they get from clicking; "Learn more" tells them nothing they didn't already know. You can test a draft CTA in the CTA tester against these criteria before it goes out, and the reasoning behind CTA wording generally is covered in the guide to writing a call to action.
Match the sign-off to the relationship
The sign-off is the last thing a reader reads before deciding how they feel about the email as a whole, so its tone should match the relationship the email is actually in. A cold email to someone who has never heard of you reads oddly if it closes "Speak soon," because nothing yet has established that familiarity. A welcome email to someone who just signed up can afford warmth, because the reader has already chosen to hear from you. A promotional email to an existing customer sits somewhere between the two: friendly, but not pretending a closeness the relationship hasn't earned yet.
Cold outreach: the subject line does the work the relationship can't
In a cold email, the subject line and opening line carry more weight than in any other email type, because there is no existing relationship supplying trust the copy can lean on. The subject line works best when it names something specific about the recipient or their situation. The CTA in cold outreach should ask for the smallest next step that makes sense, a reply or a short call, because trust hasn't been built to support a bigger ask yet.
Welcome sequences: set the expectation the rest of the sequence will meet
A welcome email's job is to tell the reader what they will get from you and how often, because the reader has just made a decision to hand over their email address and wants to know what that decision bought them. State the cadence plainly: what kind of email, roughly how frequently, and what the reader can do if it isn't what they wanted. A welcome email that opens with a discount code before it has said any of that reads as a promotional email wearing a welcome subject line, and readers notice the mismatch.
Promotional emails: state the offer without borrowing urgency it hasn't earned
A promotional email exists to move a specific offer, so the subject line and body should name the offer plainly. "Ends Sunday" is a fact worth stating if it's true; "Don't miss out" is pressure with no fact behind it, and readers who have seen enough marketing email recognise the difference within a sentence. State what the offer is, what it costs (in money, time, or effort), and by when, then give one CTA that matches those facts exactly.
PECR and consent: check the specifics with the ICO
UK marketing email sits under the Privacy and Electronic Communications Regulations (PECR), which govern whether you can send marketing email to a given address at all before a word of subject-line copy gets written. Whether consent has been given, whether a soft opt-in applies because the recipient is an existing customer being offered a similar product, and whether every email carries a working way to opt out, are all questions worth having answered before a send goes out. This guide covers how to write the email once you're allowed to send it; it is not a source of compliance advice, and questions about PECR itself belong with the ICO's own guidance.