ESP vs SMTP Relay: Which One Sends Your Business Email?
Use an email marketing platform (an ESP, in the trade's shorthand) for campaigns people subscribed to, and an SMTP relay or email API for messages your own systems send one at a time, such as receipts and password resets. A business that sells online typically sends both kinds, and a few platforms put both on one account.
The distinction matters for three reasons: the tools are built differently, they are priced differently, and US law treats their messages differently. This guide takes each in turn, with prices read on the vendors' pages on 22 September 2026.
What an email marketing platform does
An email marketing platform manages a list and the campaigns you send to it. Its core is the audience: who subscribed, what they are interested in, and who has asked to stop hearing from you.
Around that it offers the tools a person needs to write and send: a visual editor, templates, segments, automated sequences such as a welcome series, reports, signup forms, and automatic unsubscribe handling that removes people who opt out. GetResponse, Sender, EmailOctopus, AWeber, Moosend, Benchmark and Brevo are all in this group; our ranking of the best email marketing software compares them.
What an SMTP relay does
An SMTP relay delivers mail that some other system has already written. Your online store, booking system or app hands each message over — through the SMTP protocol or an HTTP API — and the relay sends it, reports whether it was delivered, and tells your system about bounces.
There is usually no list to manage and little or no editor, because the content comes from your software. What you get instead is an API, webhooks, delivery logs and templates your developers fill with data. MailerSend is a dedicated example; Brevo includes a relay alongside its marketing plans.
The same message, two routes
The easiest way to see the difference is to follow one email through each. A spring-sale announcement to 8,000 subscribers starts in a marketing platform: someone writes it in the editor, picks a segment, schedules it, and the platform sends it to everyone on the list who has not unsubscribed.
Two routes out of the same business
Top: a campaign from a marketing platform. Bottom: a receipt through a relay.
An order confirmation starts in your store the moment someone pays. The store fills a template with the order details and hands that single message to the relay, which delivers it. Eight thousand orders a month means eight thousand of those, each one triggered separately.
| Job | Email marketing platform | SMTP relay or email API |
|---|---|---|
| Who writes the message | A person, in an editor | Your software, from a template |
| Who receives it | A list or segment, at once | One customer, when something happens |
| Unsubscribe handling | Built in and automatic | Up to you, if the message is promotional |
| How it is priced | Usually by list size | Usually by emails sent |
| Typical examples | Campaigns, welcome series, announcements | Receipts, password resets, shipping updates |
How each is priced
Marketing platforms mostly charge by list size; relays charge by volume. That difference follows from the job: a campaign tool stores and manages your audience, while a relay only moves messages.
MailerSend's prices on 22 September 2026 show the relay model clearly. Its free plan includes 500 emails a month through its email API and SMTP relay. Starter costs $35 a month on monthly billing, or $31.50 a month billed yearly, for 50,000 emails a month, with extra usage at $1.30 per 1,000. Professional, aimed at agencies, costs $110 a month, or $99 billed yearly, also for 50,000 emails, and adds a dedicated account manager and longer data retention. A smaller Hobby plan was $6.30 a month billed yearly for 5,000 emails.
On the marketing side, the entry paid plans we price cost between $31.50 and $63 a month at 5,000 contacts on monthly billing. Brevo is the exception that prices marketing by volume too — Starter is listed at $9 a month from 5,000 emails.
What the law says about each
US law draws its own line between the two kinds of message. The FTC's CAN-SPAM guide treats "transactional or relationship" messages — for example, ones that complete or confirm a transaction the recipient already agreed to, or give warranty, recall, safety or security information about something they bought — as exempt from most of the Act's provisions, though they still may not contain false or misleading routing information.
Commercial messages carry the full requirements: honest headers and subject lines, an ad identified as an ad, a postal address, a working opt-out, and opt-outs honoured within 10 business days. And a mixed message counts as commercial if a reader of the subject line would take it for an ad, or if the transactional content does not come mainly at the beginning. An order confirmation stuffed with offers can become a marketing email in law.
That is why the route matters. A marketing platform handles opt-outs for you; a relay does not. If you send promotions through a relay, you are responsible for building the unsubscribe and the address footer yourself. This is general information, not legal advice.
When one account can do both
Some platforms cover both jobs. Brevo documents transactional email by API and SMTP relay on every plan, including the free one, alongside its marketing campaigns; Sender lists transactional emails on its plans; GetResponse offers them on its Enterprise plan.
One account for both means one set of domain authentication, one supplier and one bill. Two accounts — a marketing platform plus a dedicated relay — mean each tool is built for its job, and a problem in one does not stop the other. Neither is wrong. One account suits a business that values simplicity; separate accounts suit one whose receipts must never wait on a problem with its marketing. Our Brevo review looks at the one-account approach in detail.
Setting up the sending domain for both
Whichever route a message takes, it should come from a domain you own and pass authentication. Google's published sender guidelines require SPF or DKIM from every sender to Gmail accounts, and SPF, DKIM and DMARC from senders of more than 5,000 messages a day, plus one-click unsubscribe on marketing and subscribed messages.
If you use two services, both must be allowed to send for your domain: add each one's SPF include and DKIM key, and make sure your DMARC policy recognises both before you tighten it. Some businesses use a separate subdomain for each stream, which keeps the reports apart. Our bulk email services guide goes through the setup in order.
Which you need
Work backwards from the messages you send. If a person writes it and a list receives it, you need an email marketing platform. If your software writes it and one customer receives it because they did something, you need a relay. If you have both, decide whether you want one supplier or two.
- Only campaigns: a marketing platform. The guide to email marketing for small business covers what to set up first.
- Only receipts and alerts: a relay such as MailerSend (partner link), which is free up to 500 emails a month.
- Both, one supplier: a platform that includes a relay, such as Brevo.
- Both, kept apart: your marketing platform plus a dedicated relay.
Frequently asked questions
What is the difference between an ESP and an SMTP relay?
Can I send marketing emails through an SMTP relay?
Do transactional emails need an unsubscribe link?
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