---
title: What Are ESPs? Email Service Providers Explained | SendHQ
description: "ESP means email service provider: the service that sends your email. The 4 types, current pricing, Gmail and Yahoo rules, and how to choose one."
canonical: https://sendhq.cc/terms/what-are-esps
last-updated: 2026-08-26
---
# What Are ESPs? Email Service Providers Explained

TL;DR: ESPs (email service providers) are services that send email for you: they run the mail servers and IP reputation, sign mail with DKIM, process bounces and complaints, and report delivery. Marketing ESPs such as Mailchimp send campaigns to lists; transactional email APIs such as Postmark and SendHQ send messages your app triggers; Amazon SES is the raw engine underneath many of them. An ESP is not a mailbox provider like Gmail, which receives mail.

## What does ESP stand for in email?

In email, ESP stands for email service provider: a company whose software and servers send email on your behalf, so you do not run your own mail transfer agent (MTA), manage IP reputation, or process bounces yourself. An ESP is different from a mailbox provider. Gmail, Outlook.com, Yahoo Mail, and iCloud Mail host people's inboxes and decide where incoming mail lands; ESPs such as Mailchimp, Klaviyo, Brevo, SendGrid, Postmark, Mailgun, Amazon SES, and SendHQ are on the sending side and hand messages to those mailbox providers over SMTP. The acronym has other meanings that compete in search results: the National Education Association uses ESP for education support professionals, the school staff in 9 career families from custodians to paraeducators, and ESP also means extrasensory perception or a car's electronic stability program. If you landed here for email, the rest of this page is the email meaning.

## What does an email service provider actually do?

An email service provider does five jobs that are hard to do well on a single server. First, it runs the sending infrastructure: MTAs, queues, retries for temporary 4xx SMTP failures, and pools of IP addresses with established reputation. Second, it authenticates your mail by signing it with DKIM and giving you SPF and return-path records to publish in DNS, so Gmail and Yahoo can verify it. Third, it processes feedback: hard bounces (5xx), complaints from feedback loops, and unsubscribes, and it adds those addresses to a suppression list so you stop mailing them. Fourth, it gives you an interface: either a campaign editor and contact lists for marketers, or an HTTP API and SMTP relay for developers. Fifth, it reports events such as delivered, bounced, opened, clicked, and complained, by dashboard or webhook. Litmus notes that the term once meant only the marketer-facing front end, with the MTA as a separate product; today most ESPs bundle both.

## What types of ESPs are there?

There are four common types of ESP, and choosing the wrong type is the most frequent mistake. Marketing ESPs (Mailchimp, Klaviyo, Brevo, Campaign Monitor) center on contact lists, segments, a drag-and-drop editor, and campaign scheduling; they price by contacts. Transactional email APIs (Postmark, SendGrid, Mailgun, Resend, SendHQ) center on an HTTP API that your application calls when an event happens, such as a password reset or receipt, and price by messages sent. Raw cloud email services, chiefly Amazon SES, provide the sending engine and little else; you build templates, dashboards, and suppression handling yourself, in exchange for the lowest per-message price. CRM and marketing-automation suites (HubSpot, Salesforce Marketing Cloud, Braze, Customer.io) include email as one channel next to SMS, push, and lead scoring. Litmus's State of Email survey found 1,699 marketers naming more than 240 different ESPs, which shows how blurred these categories have become; many products now straddle two types.

## How much does an ESP cost?

ESP pricing follows the type. Marketing ESPs charge by stored contacts, so a list grows the bill even if you mail it rarely. Transactional APIs charge by emails sent per month, usually in plan tiers. Amazon SES, the usual floor, now publishes three plans: Essentials at $0.16 per 1,000 emails, Pro at $0.22, and Enterprise at $0.23 for the first 10 million emails a month, falling to $0.11, $0.12, and $0.13 per 1,000 above 100 million. Pro and Enterprise add a fixed fee of $105 and $500 per account, per region, per month. Many older comparison pages still quote SES at a flat $0.10 per 1,000, which no longer matches AWS's pricing page. Dedicated IP addresses, extra data retention, and inbound email processing are commonly billed separately across providers. When you compare, model your real volume and recipient count, because a provider that bills per recipient and one that bills per message diverge as soon as you send to several recipients at once.

## Which ESP rules do Gmail and Yahoo enforce?

Gmail and Yahoo enforce the same core rules no matter which ESP you use, and a good ESP makes them easy to meet. Since February 1, 2024, Google requires anyone sending more than 5,000 messages a day to Gmail accounts to authenticate with SPF and DKIM, publish a DMARC policy, align the From domain with SPF or DKIM, keep the spam rate reported in Postmaster Tools below 0.30%, and support one-click unsubscribe on marketing mail. Yahoo's sender best practices match: spam rate below 0.3%, a DMARC policy of at least p=none, From-domain alignment, the RFC 8058 List-Unsubscribe-Post method, and honoring unsubscribes within 2 days. These obligations sit with the sender, not the ESP. The ESP signs the mail and processes the unsubscribe request, but you publish the DNS records on your domain and you own the complaint rate. That is why moving to a different ESP rarely fixes a deliverability problem on its own.

## Which ESP type fits your use case?

Choose an email service provider by the job, not the brand. Use this test. If marketers build and schedule newsletters to a list, pick a marketing ESP. If your application sends messages triggered by user actions, pick a transactional API, and check four things: an idempotency mechanism so retries do not send duplicates, delivery events by webhook or API, suppression handling, and clear limits per request. If cost per message dominates and you have engineers to build tooling, use Amazon SES directly. If you also need to receive mail, for support replies, inbound parsing, or AI agents that read and answer email, check that inbound is a first-class feature rather than a separate product. Separate transactional and marketing traffic onto different subdomains (for example mail.example.com and news.example.com) so a bad campaign does not slow password resets. Finally, confirm that the provider lets you export suppression lists, because switching ESPs without them re-mails people who bounced or complained.

## How does SendHQ fit as an ESP?

SendHQ is a transactional email API with inbound email built in, aimed at product teams and AI agents. It sends through Amazon SES by default behind a pluggable provider layer, and its public API is compatible with Resend's. A single request can address up to 100 combined To, Cc, and Bcc recipients, and POST /emails/batch accepts up to 100 separate messages. Every send can carry an Idempotency-Key of up to 200 characters, so a retried request replays the original response instead of sending twice. Each message moves through explicit statuses (queued, sent, delivered, bounced, complained, failed), so 'accepted by the provider' is never confused with 'delivered'. On the receiving side, inboxes on your verified domain parse incoming mail and thread it with your outbound messages. SendHQ is not a marketing ESP: it has no campaign editor or contact-list builder, so if your main job is newsletters, a marketing ESP is the better fit.

```
curl https://sendhq.cc/api/v1/emails \
  -H "Authorization: Bearer $SENDHQ_API_KEY" \
  -H "Content-Type: application/json" \
  -H "Idempotency-Key: welcome-user-8812" \
  -d '{"from":"Acme <hello@mail.acme.dev>","to":["new.user@example.com"],
       "subject":"Welcome to Acme","text":"Your account is ready."}'
```

## Questions teams ask

**What is the difference between an ESP and a mailbox provider?**

An ESP sends email for a business; a mailbox provider such as Gmail, Outlook.com, or Yahoo Mail hosts recipients' inboxes and filters incoming mail. Deliverability is the relationship between the two: the ESP hands mail over, and the mailbox provider decides whether it reaches the inbox.

**Is Gmail an ESP?**

Not in the industry sense. Gmail is a mailbox provider for people's inboxes. Google Workspace lets you send ordinary mail, but it caps daily sending and has no campaign tools or sending API built for application volume, so businesses use an ESP for bulk or automated email.

**What is the difference between an ESP and an SMTP relay?**

An SMTP relay accepts mail over SMTP, usually on port 587, and passes it onward. Most ESPs offer an SMTP relay as one interface, alongside an HTTP API, templates, suppression lists, and event reporting. A bare relay gives you transport without those management features.

**Do I need an ESP for transactional email?**

For any real volume, yes. Sending from your own server means managing IP reputation, DKIM signing, bounce processing, and retries yourself, and new IPs are often throttled by Gmail and Microsoft. A transactional API handles that and gives you delivery events.

**What does ESP mean in schools?**

In education, ESP means education support professional: school staff such as paraeducators, bus drivers, custodians, food service, clerical, and technical workers. The National Education Association groups them into 9 career families. It is unrelated to the email meaning.

## Primary sources

- [What is an ESP?](https://www.litmus.com/blog/what-is-an-esp) — Litmus
- [Amazon SES pricing](https://aws.amazon.com/ses/pricing/) — Amazon Web Services
- [Email sender guidelines](https://support.google.com/a/answer/81126) — Google
- [Sender best practices](https://senders.yahooinc.com/best-practices/) — Yahoo
- [Sending email](https://sendhq.cc/docs/sending.md) — SendHQ

## Continue learning

[transactional email vs marketing](https://sendhq.cc/guides/transactional-email-vs-marketing.md) [transactional email api](https://sendhq.cc/guides/transactional-email-api.md) [amazon ses cost](https://sendhq.cc/guides/amazon-ses-cost.md) [email api pricing](https://sendhq.cc/guides/email-api-pricing.md) [smtp relay service](https://sendhq.cc/guides/smtp-relay-service.md)