What Is Email Marketing? How It Works and the Rules to Follow

What Is Email Marketing?

Email marketing is using email to build a relationship with people who have chosen to hear from you. It covers newsletters, welcome messages, product announcements, offers, and automatic emails triggered by what someone does, such as signing up or leaving items in a basket.

What makes email different from social media or search is ownership. Followers on a social platform can disappear if the platform changes its rules, and search traffic can fall when Google changes its results. An email list is a direct line to people who asked to hear from you, and it moves with your business wherever you go.

Email is also flexible. The same list can receive a weekly newsletter, a launch announcement and an automatic welcome series, each written for a different moment in the customer’s relationship with the business. That direct line also brings responsibility. Email is regulated in most countries, and email providers filter heavily against unwanted messages, so good email marketing is built on permission and usefulness rather than volume.

How Email Marketing Works

Most email programmes follow the same simple cycle.

1. Earn the sign-up. Offer something worth giving an email address for: a useful newsletter, a guide, a discount or early access.
2. Welcome new subscribers. Send what you promised straight away and tell people what to expect and how often.
3. Send regular value. Helpful content builds trust and keeps people opening your emails.
4. Make relevant offers. Promote products or services occasionally, matched to what subscribers have shown interest in.
5. Measure and improve. Watch what people click, reply to and buy, and adjust.

The Main Types of Marketing Email

Different emails do different jobs, and most businesses use several.

Transactional emails are treated differently by the law, because they confirm something the customer started. The US Federal Trade Commission notes that messages which are strictly transactional or relationship messages don’t have to offer a marketing opt-out in the same way.¹ Adding heavy promotion to them can change that, so keep them focused.

The Rules: Email Marketing Law in the US and UK

Email marketing is legal almost everywhere, but it comes with rules, and breaking them can be expensive. This is a summary for education, not legal advice.

United States: the CAN-SPAM Act

The US Federal Trade Commission’s compliance guide sets out the main requirements of the CAN-SPAM Act:¹

• Don’t use false or misleading header information, such as the “From” name and address.
• Don’t use deceptive subject lines.
• Identify the message as an advertisement.
• Include a valid physical postal address.
• Tell recipients how to opt out, and honour opt-outs within 10 business days.
• Monitor anyone who sends email on your behalf; you remain legally responsible.

The FTC says each separate email that breaks the law can bring penalties of up to $53,088, and that there is no exception for business-to-business email.¹

United Kingdom: PECR

UK rules are stricter on permission. The Information Commissioner’s Office explains that you generally must not send marketing emails to individuals without their specific consent, such as ticking an opt-in box.² There is a limited exception, the “soft opt-in”, for existing customers who bought or negotiated to buy a similar product from you and were given a clear chance to opt out when you collected their details and in every message.²

The ICO notes that the rules differ for emails to companies: you can email corporate bodies, but sole traders and some partnerships are treated as individuals.² If you email people in other countries, check their rules too: the strictest set that applies is the one to follow.

Gmail’s Sender Rules: Getting Into the Inbox

Even a legal email can end up in spam if it doesn’t meet the mailbox providers’ technical rules. Since February 2024, Google has set requirements for anyone sending to Gmail accounts:³

Google recommends keeping spam complaints below 0.10% and never reaching 0.30%.³ In practice, if a few people in every thousand mark your emails as spam, deliverability suffers. Most email marketing services handle the technical set-up, but you need to add the authentication records to your domain, and only you can keep complaints low by emailing people who actually want to hear from you.

What to Measure

The numbers that matter most are the ones closest to real interest and real results.

Why open rates became unreliable

Open rates were once the headline email metric. That changed in 2021, when Apple introduced Mail Privacy Protection in its Mail app. Apple said the feature stops senders using invisible pixels to collect information, hides whether an email was opened, and masks the user’s IP address.⁴ Because senders can no longer reliably tell whether a person actually opened an email, many marketers now treat open rates as a rough guide at best and focus on clicks, replies and sales.

A Worked Example: A Small Online Shop

Here is how the numbers might look for an illustrative small business. The figures are made up for the illustration, not industry benchmarks.

The shop’s real lesson isn’t the $300. It is that every number can be improved: more relevant subscribers raise the click rate, a clearer product page raises the purchase rate, and a better welcome series builds trust before the offer. Small gains at each step multiply: raising both the click rate and the purchase rate by a fifth would lift revenue from the same email by more than 40%. Whether $300 is worthwhile depends on the shop’s profit margin, not just its revenue.

Writing an Email People Want to Read

The best marketing emails read like a helpful note from someone you trust, not an advert. A few principles make the biggest difference:

1. One email, one main point. A single clear message and a single main link beat a crowded layout.
2. An honest, specific subject line. Say what is inside. Curiosity is fine; deception breaks US law.¹
3. A recognisable sender name. People open emails from names they know, so use the same one every time.
4. Useful before promotional. Give readers something worth their time, then make the offer.
5. Short paragraphs and a clear button. Most people scan, often on a phone.
6. A human reply address. Inviting replies builds trust, and replies tell you what readers want.

Segmentation: Sending the Right Email to the Right People

Segmentation means splitting your list into groups so each gets more relevant emails. A shop might send different emails to first-time buyers and regular customers; a publisher might let readers choose topics. More relevant emails usually mean more clicks, fewer unsubscribes and fewer spam complaints.

Simple segments are often enough to start: what people signed up for, whether they have bought, and how recently they clicked. Use only data people would expect you to use, and follow your privacy policy. Asking subscribers directly what they want, through a one-question survey or topic preferences, is both respectful and effective.

Choosing an Email Service

Use a dedicated email marketing service rather than your normal inbox. These services manage sign-up forms, unsubscribes, legal footers, sending at scale and the reporting you need. When comparing them, check the price as your list grows, how easily you can export your list, whether they help with authentication records, and whether their templates work well on phones.

How to Start an Email List the Right Way

A good list is built, never bought. Bought lists break UK rules for individuals, damage deliverability and rarely contain people who want your emails.²

1. Choose an email service that handles unsubscribes, authentication and the legal footer for you.
2. Write a clear promise: what people get and how often, such as “one explainer every Thursday”.
3. Add a sign-up form to your website where it is relevant, such as the end of a helpful article.
4. Use clear consent: an unticked opt-in box or an obvious sign-up button, with a link to your privacy policy.
5. Send a welcome email immediately and deliver what you promised.
6. Keep your promise. Send on the schedule you said, with the content you said.

Automation: Emails That Send Themselves

Automated emails are written once and sent whenever someone takes a particular action. They often perform well because they arrive at the moment they are most relevant. Common examples:

• Welcome series when someone subscribes.
• Abandoned-basket reminders when someone leaves items unpaid.
• Post-purchase emails with tips on using the product, then a request for a review.
• Re-engagement emails asking inactive subscribers whether they still want to hear from you.

Review automated emails every few months. Prices, policies and links change, and an automated email that nobody has looked at for a year can quietly give customers wrong information. Removing subscribers who never respond to a re-engagement email also helps, because sending to people who ignore you can count against your deliverability.

Common Email Marketing Mistakes

Most problems come from treating subscribers as a mailing list rather than as people.

• Sending only offers. If every email is a sale, people stop opening.
• Hiding the unsubscribe link. It increases spam complaints, which is worse for deliverability than an unsubscribe.
• Misleading subject lines. They break US law and lose trust.¹
• Emailing old or bought lists. High bounce and complaint rates can push all your emails into spam.
• Ignoring mobile. Emails that are hard to read on a phone get deleted.
• Measuring only opens. Clicks, replies and sales tell you far more.

Email Marketing and Search

Email and SEO work best together. Search brings new readers to your site; email turns some of them into a returning audience you can reach directly. As more searches end without a click, partly because of AI answers at the top of results, that direct audience becomes more valuable. Pages written for the right search intent bring in readers who are genuinely interested, and those are exactly the people most likely to subscribe and stay.

Key Terms Explained

These are the terms you will see most often in email marketing tools and reports.

The Bottom Line

Email marketing works when people asked to hear from you and are glad when you do. Get clear permission, follow the rules where your subscribers live, meet the mailbox providers’ technical standards, and judge success by clicks, replies and sales rather than opens. Done well, a list is one of the most valuable assets a business can build. For more, read about search intent and SEO, or browse more marketing explainers.

To judge how your campaigns perform, see our explainer on email open rate and the metrics that matter more.

Email is one channel among several. Our explainer on digital marketing shows how they fit together.

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