What is email marketing? Send legally, reach the inbox: SPF, DKIM, DMARC and unsubscribe

Email marketing là gì? Email marketing là việc gửi thông điệp thương mại, thường cho một nhóm người, qua email để chăm sóc khách hàng hiện tại và cũ, khuyến khích mua lại, thu hút khách mới hoặc thuyết phục mua ngay. Phần khó của email marketing ngày nay không còn là viết tiêu đề hay mà là bảo đảm thư vào được hộp thư đến và đúng luật: từ 1/2/2024 Gmail và Yahoo, theo hướng dẫn của Google (đọc ngày 23/10/2026), yêu cầu người gửi xác thực tên miền, giữ tỉ lệ báo thư rác dưới 0,3% và (với người gửi nhiều) hỗ trợ huỷ đăng ký một chạm. Còn ở Việt Nam, Nghị định 91/2020/NĐ-CP quy định riêng về email quảng cáo: phải có sự đồng ý trước, gắn nhãn [QC] hoặc [AD], có chức năng từ chối nhận. Bài này tổng hợp các yêu cầu đó từ văn bản gốc và hướng dẫn chính thức, kèm quy trình thực hành cho doanh nghiệp nhỏ.

Messages arriving in the inbox need three layers: recipient consent, domain authentication, and easy unsubscribe. Diagram: OPCAIOS.

What is email marketing and what types are there?

Email marketing is the act of sending commercial messages, usually to a group of people, by email; according to Wikipedia (read on October 23, 2026), in the broader sense it includes advertising, sales pitches, or fundraising appeals. The purpose is typically to increase engagement with current and past customers, encourage loyalty and repeat purchases, attract new customers, or persuade immediate purchase. There are three common types:

1. Transactional email: sent when a customer takes action, such as order confirmation, password reset, or receipts.
2. Direct email: solely for delivering promotional messages, such as offers or product catalogs.
3. Newsletters and nurture sequences: sent regularly or in a sequence after a customer subscribes.

For example, an order confirmation email is a transactional email, while a holiday promotion email is a direct email. Sharing the goal of customer engagement, email marketing fits into the bigger picture of content marketing.

What are the conditions for emails to reach the inbox of Gmail and Yahoo?

From February 1, 2024, first, all email senders to Gmail must meet Google's basic requirements, and senders of more than 5,000 messages per day must meet additional ones. According to Google's "Email sender guidelines" page (read on October 23, 2026):

Requirement	All senders	Senders of more than 5,000 messages per day
Authentication	SPF or DKIM	SPF, DKIM and DMARC (policy can be none)
DNS	Valid forward and reverse (PTR) DNS records	Same
Connection	Use TLS	Same
Spam ratio	Below 0.3% in Postmaster Tools	Same
Format	Per RFC 5322; do not spoof Gmail From	Same, plus alignment with SPF or DKIM
Unsubscribe	Recommended	Mandatory one-click unsubscribe with clear unsubscribe link in message

Google recommends keeping the spam ratio below 0.10% and never reaching 0.30% or higher. Google also says they do not track open rates and cannot verify open rates reported by third parties. Additionally, the 5,000 messages per day limit is the threshold dividing two groups of senders.

What does Yahoo require from email senders?

Yahoo requires email senders to authenticate their domain, keep spam ratios low, and allow easy unsubscribe, applying from February 2024 and rolling out gradually. According to Yahoo's sender guidelines (read on October 23, 2026), the requirements are similar to Google's. All senders need SPF or DKIM, spam ratio below 0.3%, and valid forward and reverse DNS. High-volume senders need three additional things:

1. SPF together with DKIM.
2. DMARC minimum p=none.
3. One-click unsubscribe via List-Unsubscribe header; POST method per RFC 8058 is recommended.

Additionally, the message must have a clear unsubscribe link in the content and unsubscribe requests must be processed within 2 days. For example, a business sending 6,000 messages per day to Gmail falls into the high-volume sender group, so it needs DMARC and one-click unsubscribe under both Google and Yahoo. Yahoo also recommends not buying lists and not pre-checking consent boxes.

SPF, DKIM, DMARC, and One-Click Unsubscribe – What Do They Mean?

SPF, DKIM, DMARC, and one-click unsubscribe are four technical terms behind the requirements from Gmail and Yahoo, according to Google (read 23/10/2026) and Yahoo. Specifically:

1. SPF: a DNS record declaring which mail servers are allowed to send mail on behalf of your domain.
2. DKIM: a digital signature attached to mail so the recipient can check that the mail has not been altered and truly comes from your domain.
3. DMARC: a policy telling the recipient what to do when SPF or DKIM doesn't match, along with reports sent back to you (reporting tag). The policy to start with can be none, which means monitoring only.
4. One-Click Unsubscribe (RFC 8058): mail with two headers List-Unsubscribe and List-Unsubscribe-Post: List-Unsubscribe=One-Click. When the recipient clicks the unsubscribe button right in the mail app, your server receives a POST request and must unsubscribe them.

Most email service providers (ESP) guide you on setting up SPF and DKIM for your sending domain. For example, before sending to Gmail, use your own domain instead of a free address, then check the records using your provider's tool.

What Does Decree 91/2020/NĐ-CP Say About Advertising Email?

Decree 91/2020/NĐ-CP (effective from 1/10/2020, replacing Decree 90/2008 and 77/2012) is a document regulating the sending of advertising messages, email, and calls in Vietnam, according to Decree 91 (issued by the Government of Vietnam in 2020). For example, a store sending a promotional email must have prior consent and the label [QC] at the beginning of the subject line. Below are three key points about advertising email that we read directly in the document (read 23/10/2026):

• Prior consent required (Article 11): you may only send advertising email when the user has given prior consent, through methods such as filling out and confirming a registration form on the advertiser's website or social media. The advertiser must keep registration information, opt-out requests, and opt-out confirmations for a minimum of 1 year.
• Labeling (Article 18): every advertising email must have the label [QC] or [AD] in the first position of the subject line.
• Advertiser information (Article 19): including name, phone number, email address, geographic address, website or social media (if any), placed right before the opt-out section.

What Are the Opt-Out Function and Frequency Limits in Decree 91?

Three other points, according to Decree 91 (read 23/10/2026) and Articles 13, 17, 20, are the opt-out function, frequency limits, and content. For example, a sales email needs an opt-out function at the end of the email.

• Opt-Out Function (Article 20): placed at the end of the email and clearly stated; the user has the right to opt out of all products from the advertiser, and when needed to opt out of individual products or groups. Forms of opt-out include via website or social media, by email, and by phone. Upon receiving an opt-out request, you must immediately send a confirmation message (not containing advertising) and cease sending.
• Frequency Limits (Article 13): do not send more than 3 advertising emails to one address within 24 hours, unless otherwise agreed.
• Content and Subject Line (Article 17): the subject line must match the content; content must comply with advertising law.

What Are Foreign Laws and the Limits of This Summary?

This is a summary for informational purposes, read from the 2020 document; the document may have been amended, so please check the currently effective text or consult a lawyer. For customers in other countries, there are separate laws: for example, according to the FTC (USA) on CAN-SPAM, which we only read through search excerpts, commercial email needs accurate header information, subject lines that do not mislead, a valid postal address, clear opt-out, and opt-out processing within 10 business days; please check the FTC website directly.

Email marketing process for small businesses

1. Build your list with clear consent: signup form on your website with clear language about what people will receive and how often. Yahoo advises against buying lists and against pre-checking consent boxes.
2. Confirm addresses before adding them: Google advises confirming each email address before signup, and periodically asking recipients if they want to keep receiving.
3. Send exactly what you promised: Yahoo reminds you not to switch weekly newsletter subscribers to daily emails.
4. Set up domain authentication: SPF, DKIM, DMARC (start with p=none) before sending bulk mail; monitor reports.
5. Write short emails with one goal: one call to action; label [QC] when it's an ad; sender info and unsubscribe link at the bottom.
6. Make unsubscribing as easy as possible, and stop sending right away. Google says letting users stop receiving mail can improve open rates, click rates, and sending effectiveness.
7. Measure the right things: spam complaint rate (Postmaster Tools), unsubscribe rate, click rate, conversion. Don't just look at open rate, because Google says they don't track and can't verify this number.
8. Clean your list: Google advises considering removing people who don't open or read your emails.

Is it safe to use AI to write marketing emails?

AI helps draft quickly and test many variations, but rules about consent, ad labeling, truthful content, and unsubscribing don't change. Have someone review before sending bulk mail, check every number and promise, and keep your brand voice consistent; see Vibe marketing là gì? on the risk of bland content, and n8n, Make, Zapier if you want to automate your sending process. If you need to store customer lists and care history, see HubSpot là gì?.

OPCAIOS's experience sending care emails

OPCAIOS uses care email sequences in the admin area: sends through a private email service's SMTP server, has an unsubscribe link in each email, click links are signed to prevent spoofing, tracks opens and clicks, and keeps a separate rejection list so we don't send to people who've unsubscribed. Each failed send is retried up to 3 times. We've also noted two gaps compared to best practice: the unsubscribe link is currently a confirmation page via GET, doesn't yet support one-click unsubscribe per RFC 8058 via POST; and the domain's DMARC policy is in monitoring mode (p=none). At our current sending scale, much lower than Google's 5,000 emails per day threshold, bulk sender requirements don't yet apply, but these are things to improve before scaling up. We're not publishing open or click rates in this post.

Read more: Running a paid newsletter business; The "Cold Outreach without spam" technique.

Frequently Asked Questions

What is email marketing?

It is sending commercial messages via email, usually to a group of people, to care for customers, encourage repeat purchases, attract new customers, or make sales.

What does Gmail require for email marketing?

From 1/2/2024: everyone sending needs SPF or DKIM, valid forward and reverse DNS, TLS, and a spam rate below 0.3%. Senders of more than 5,000 emails per day need to add DMARC, matching domain names, and one-click unsubscribe with a clear link in the email.

What is needed to send advertising emails in Vietnam?

According to Decree 91/2020/NĐ-CP: prior consent from the recipient, tag [QC] or [AD] at the start of the subject line, information about the advertiser, an opt-out function at the end of the email and stop sending immediately if rejected, no more than 3 advertising emails per day to one address unless agreed otherwise. Check the current text and consult a lawyer.

Is open rate important?

Less than you think. Google says they do not track open rates and cannot verify numbers reported by third parties; spam rate, unsubscribe rate, click rate, and conversion rate are more reliable indicators.

Where to start with email marketing?

Starting email marketing means building three foundational things before sending in bulk. First, a signup form with clear consent, stating clearly what the recipient will receive and the frequency. Next, a sending domain with SPF and DKIM installed, then DMARC starting at p=none, per Google and Yahoo (read 23/10/2026). Finally, a short welcome email with a clear unsubscribe link. For example, a small shop can collect addresses through a form on its website, send a welcome email, then only send a newsletter once a week as promised. For advertising emails in Vietnam, remember to tag [QC] or [AD] per Decree 91/2020/NĐ-CP. To learn how to build customer care workflows and automate with AI, see OPCAIOS's AI application training course.

Sources: Google's "Email sender guidelines" page (Gmail Help) and Yahoo's sender guidelines, read directly on 23/10/2026; Decree 91/2020/NĐ-CP text (full PDF) read directly Articles 11, 13, 17 to 20, 35; Wikipedia "Email marketing"; FTC CAN-SPAM information obtained via search excerpt because the page would not load automatically. Configuration experience is from OPCAIOS. Not legal advice; laws and provider requirements may change.