This guide is for newsletter writers, bloggers and small-business owners who send email and want more people to open it, without resorting to tricks that cost trust. It covers how to write a subject line and preview text that fit the email inside, how to handle length on mobile, personalization, legal and deliverability limits, A/B testing, and why open rates need to be read with care.
Contents
Start with a simple rule: the subject is a promise
A subject line works best when it tells the reader what they will get and the email then delivers it. That is also close to what the law requires. The FTC’s CAN-SPAM compliance guide lists “Don’t use deceptive subject lines” as one of its core requirements and explains that the subject line must accurately reflect the content of the message. Google’s email sender guidelines say much the same thing: message headers and content should be accurate and not misleading or deceptive.
In practice, write the email first, then write the subject line from what is actually in it. If the email is a roundup of three articles, a subject that promises one dramatic revelation sets the wrong expectation, even if the revelation is technically in there somewhere.
Ways to make the subject match the content
- Name the main topic or benefit in plain words (“Three ways to reuse old blog posts”) rather than a vague teaser.
- If the email has one clear offer, deadline or event, say so in the subject.
- Be direct rather than trendy. Mailchimp’s subject line guidance suggests describing benefits or specific deals instead of generic seasonal slogans that lack a hook.
- Never start a subject with “Re:” or “Fwd:” unless the message really is a reply or forward. Google’s sender guidelines call this out directly.
Subject line length and mobile
Many people read email on a phone, where less of the subject line is visible. Mailchimp’s help page on subject line best practices recommends no more than 9 words and 60 characters, and also suggests no more than 3 punctuation marks and no more than one emoji at a time. Treat those as one platform’s guidance, not a universal law: different email apps and screen sizes cut subjects off at different points, and no official page promises a single cut-off that applies everywhere.
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The practical takeaway is to put the most important words first. If the subject is trimmed on a small screen, the reader should still see what the email is about.
Preview text (preheader): the second subject line
Preview text is the snippet shown beside or under the subject line in the inbox. Mailchimp’s help page explains that if you do not set it, email clients usually pull in the start of your message, which can mean “View in browser” links or even stray HTML code. That is a wasted line of attention.
How to set it in Mailchimp
In a regular campaign, open the email checklist and click the Subject section. Enter your subject and preview text in the matching fields, then click Save.
What to put in it
- Keep it short. Mailchimp notes that most email clients cap how much preview text they show, and it does not give a single character limit.
- Put the most important detail first, such as the offer or the news.
- Use it to support the subject, not repeat it. If the subject cannot hold everything, the preview text can carry the second half of the thought.
The same honesty rule applies. Preview text that promises something the email does not contain misleads just as much as a bad subject line.
Personalization that helps rather than annoys
Mailchimp’s guidance says you can use merge tags to add a recipient’s name or location, and that personalization is shown to increase open rates for most users. It works best alongside targeted messages such as birthday offers or post-purchase follow-ups, where the detail is relevant to the reader. A first name pasted into every subject line quickly becomes background noise, and a missing or wrong merge value (“Hi , “) looks careless. Set a fallback value for contacts with empty data.
Relevance often beats name-dropping. Sending different subjects to different groups is easier once you have sorted subscribers by interest; our guide to email list segmentation for small newsletters shows how to start.
Avoid spammy and deceptive patterns
| Pattern | Why it causes trouble | Better approach |
|---|---|---|
| Fake “Re:” or “Fwd:” prefix | Google’s sender guidelines say not to use these unless the message is a real reply or forward. | Use a plain subject that describes the email. |
| Promising content that is not in the email | The FTC says the subject line must accurately reflect the content of the message. | Write the subject after the email is finished. |
| Heavy punctuation and special characters | Mailchimp warns too many punctuation marks can make an email look like spam. | Keep punctuation minimal. |
| A subject that disguises a promotion as a routine message | The FTC guide treats an email as commercial when its content is promotional, whatever the subject suggests, so all CAN-SPAM requirements apply. | Be upfront that it is an offer or newsletter. |
On the stakes: the FTC states that each separate email in violation of the CAN-SPAM Act can be subject to penalties of up to $53,088, a figure it says was updated in January 2024 for inflation. This article is not legal advice, and CAN-SPAM has other requirements (such as honest sender information and an opt-out) beyond the subject line, so read the full guide if you send commercial email to US recipients.
Deceptive subjects also hurt deliverability. Google asks all senders to keep their spam rate in Postmaster Tools below 0.3%, with a stricter target of staying under 0.10%. Readers who feel tricked tend to mark mail as spam, and Google says anyone sending more than 5,000 messages a day to Gmail accounts must support one-click unsubscribe in marketing and subscribed messages. Authentication matters too; see our SPF, DKIM and DMARC setup guide for newsletter senders.
A/B test subject lines properly
Guessing which subject will work is less reliable than testing. According to Mailchimp’s A/B testing help page, an email test changes one variable at a time (subject line, From name, content or send time) and can have up to three variations. Each contact is randomly given one combination only. Subject line tests can compare phrasing, offers or emoji.
- Pick one question, for example “Does a specific benefit beat a question?”
- Change only the subject. If the content also differs, you cannot tell what caused the result.
- Choose how the winner is decided. Mailchimp offers an automatic winner by open rate, click rate or revenue, or a manual choice based on your report.
- Record what you learned and apply it to later sends.
Plan limits apply: Mailchimp says A/B tests depend on your plan and multivariate tests need a Standard plan or higher. Treat results from very small lists cautiously.
Why open rates are less reliable now
Open tracking relies on a tiny image loading when the email is opened. Mailchimp’s help page notes this means open rates are not 100% accurate: if images are blocked, the open goes unrecorded. It also names Apple Mail Privacy Protection as a type of bot activity that can falsely inflate open and click metrics, so some recorded opens may not reflect a real person reading. Mailchimp recommends excluding Apple MPP and other bot activity from your reports; its separate help article on bot filtering explains how.
This matters for subject line testing, because the open rate is the main measure of a subject line. Use it as a rough comparison within the same list and period, not as a precise score, and look at clicks and replies alongside it.
If you are just starting out, a good welcome email is a strong place to practise these habits; see our guide to building an email welcome sequence for a newsletter.
Key takeaways
- The subject line must accurately reflect the email’s content; the FTC and Google both say so.
- Put the key words first and check how the subject looks on a phone. Mailchimp suggests 9 words and 60 characters or fewer.
- Always set preview text; it is a second chance to say what the email offers.
- Personalize only when the detail is relevant and the data is clean.
- A/B test one variable at a time, and read open rates cautiously because of Apple MPP and other bot activity.
FAQ
How long should an email subject line be?
Mailchimp recommends no more than 9 words and 60 characters. Display limits vary by email app and device, so put the important words first and test on a phone.
Is it illegal to write a catchy subject line?
No. The CAN-SPAM rule is that a subject line must not be deceptive and must accurately reflect the message content. A curious or benefit-led subject is fine if the email delivers on it.
What happens if I do not set preview text?
Email clients typically show the start of your message instead, which according to Mailchimp can include “View in browser” links or HTML code.
Can I still trust open rates?
Only partly. Mailchimp says Apple Mail Privacy Protection can falsely inflate opens and that image-based tracking is not fully accurate. Compare tests within your own list, filter bot activity where your platform allows, and weigh clicks too.
