The email APIs and SMTP relays apps use to send receipts, password resets and order updates, ranked by how many customers or users each one says it serves.
Twilio SendGrid is the largest transactional email service, used by 12M+ developers and delivering 205+ billion emails a month. Brevo is second with 600,000+ customers and a free plan of 300 emails a day, and Mailgun is third with 150,000+ businesses. For the lowest cost at scale, Amazon SES charges $0.10 per 1,000 emails, and Postmark is a favorite for fast app email.
How it's ranked: Customers, businesses, developers or users each vendor says it serves, as published on its own website (read November 2026). Counts are each company's own claims and measure different things (SendGrid counts developers, Mailtrap counts monthly active users, the others count customers or businesses), so treat them as a guide to scale, not an audit. Amazon SES, Postmark, Resend and MailerSend publish no exact customer count and are ranked last, in no particular order of size.
Twilio · Email API and SMTP relay at very high volume
12M+ developers
The biggest and most battle-tested email API.
Twilio SendGrid says 12M+ developers have used its Email API, and that it delivers 205+ billion emails monthly, by far the largest numbers on this list. You can send through RESTful endpoints or an SMTP relay, with SPF, DKIM, DMARC and TLS encryption supported. Twilio publishes a 99% deliverability rate, a 99.99% uptime SLA and a 1.9 second median delivery speed. Extras include dynamic templates, real-time email address validation at 350 validations per second, deliverability insights and analytics, and dedicated IPs on higher tiers. Shopify uses it for payment and shipping alerts across its 1.7 million merchants. The free trial lets you send 100 emails per day for 60 days before upgrading. It fits product teams that expect to grow into very large volumes and want a provider that has already been tested at that size.
Best forApps that send at huge scale and want a long-proven API
Brevo · Transactional email, SMS and WhatsApp API with SMTP relay
600,000+ customers
The best free plan, with EU data hosting.
Brevo says 600,000+ customers around the world trust it, and its transactional product sends order confirmations, receipts and other critical messages by email, SMS and WhatsApp from one platform. It offers a REST API with official SDKs in 7 languages (PHP, Node.js, Ruby, Python, Go, Java and C#) plus an SMTP relay for platforms such as WordPress. Batch sending handles up to 1,000 emails per API request, idempotency keys stop duplicate sends, attachments can be up to 25 MB and data is hosted in France and Germany. Real-time webhooks report opens, clicks, bounces and deliveries, and inbound parsing turns replies into a two-way exchange. The free plan allows 300 emails a day (9,000 a month) with no credit card, and the Starter plan starts at $9 a month for 5,000 emails.
Best forSmall businesses that want a generous free plan and EU hosting
Skip it ifYou only need a bare-bones developer API
Mailgun says over 150,000 businesses use it, including 8 of the 10 largest tech companies in the world. Its core is an email API and SMTP service to send, receive and track email, and it adds tools to preview and test emails before they go out and to clean your list via API or bulk upload. Inbound routes let your app receive and process incoming mail. The free plan includes 100 emails a day, 1 custom sending domain and 1 day of log retention. Paid plans start at $15 a month for 10,000 emails on Basic, and the Foundation plan costs $35 a month for 50,000 emails with 5 days of log retention. It fits engineering teams that want a mature, well-documented API with code examples in many languages and room to scale.
Best forDeveloper teams that want sending, testing and list validation in one place
Skip it ifYou need long log retention on a small budget
Mailtrap · Email sending API, SMTP and testing sandbox
150,000+ monthly active users
Best for testing and sending in one tool.
Mailtrap says it is trusted by 150,000+ monthly active users and draws on 15 years in email. It started as a testing tool and now runs 3 products on one platform: Live Sending for transactional and bulk email via API or SMTP with logs and delivery analytics, a Sandbox that catches test emails before they reach real inboxes, and Marketing. The sandbox accepts SMTP on ports 25, 465, 587 or 2525. On the pricing page, the free sending plan includes 4,000 emails a month (150 a day) with 3 days of logs, and Basic starts at $15 a month for 10,000 emails. Business plans add up to 100 users. It suits development teams that want to test templates in staging and then send the same emails in production with one vendor.
Best forTeams that want to test emails safely before sending them for real
Skip it ifYou need marketing automation more than app email
Sinch · Email API and SMTP relay for transactional and marketing email
100,000+ customers
Best for developers and marketers together.
Mailjet says 100,000+ customers around the world trust it. Its RESTful Email API and SMTP relay handle both marketing and transactional email at any volume, with SDKs for PHP, Python and Node, sandbox testing before launch and webhooks for real-time event tracking. Deliverability features include dedicated IP addresses and domain authentication, and it runs on EU-based data centers with a 99.99% SLA. It also offers 100+ ready-to-use integrations. One customer says it sends about 10M emails a month through Mailjet. The free plan allows up to 6,000 emails a month with a maximum of 200 a day, and the Starter plan costs $9 a month for 8,000 emails with no daily limit. It fits companies where developers send receipts and marketers send newsletters from the same account.
Best forTeams that want developers and marketers working in the same tool
SMTP2GO · SMTP relay and email API with delivery tracking
40,000+ businesses
The simplest reliable SMTP relay.
SMTP2GO says over 40,000 businesses worldwide use it for rock-solid email delivery, and it has been on the Deloitte Fast500 5 years in a row. It is built around a reliable SMTP relay with an API, running on multiple data centers so email keeps flowing during major outages. It avoids the low hourly or daily caps common with ISP and web host SMTP, handles SPF and DKIM automatically, tracks every email to its destination and reports bounces, spam complaints and unsubscribes. Alerts flag problems such as a sudden rise in bounce rate, and round-the-clock support is free. The free plan covers up to 1,000 emails a month, and the Starter plan costs $10 a month for 10,000 emails. It suits small firms and IT admins who need devices, websites and apps to send mail reliably.
Best forSmall businesses and IT teams that need a dependable SMTP relay
Skip it ifYou want a modern developer-first API experience
Amazon Web Services · Cloud email service with pay-as-you-go pricing
No customer count published (1 trillion+ emails a year)
The cheapest per email at scale.
Amazon Simple Email Service does not publish a customer count, so it ranks below the vendors that do, but AWS says it transacts more than a trillion email each year for customers such as Netflix, Duolingo and Amazon Retail. It sends transactional email such as purchase confirmations, shipping alerts and order updates, as well as marketing and newsletter email, and integrates into any application. You can use dedicated, shared or owned IP addresses, and it provides sender statistics, deliverability tools and Mail Manager for incoming email. Pricing is pay for what you use: on the pricing page, a la carte sending costs $0.10 per 1,000 emails, and a standard dedicated IP costs $24.95 a month. New AWS customers get up to $200 in Free Tier credits. It fits teams already building on AWS that want very low unit costs.
Best forTeams already on AWS that want the lowest cost per email
Skip it ifYou want hands-on support and a friendly dashboard
Postmark · Transactional email API and SMTP focused on speed
No exact count published ("thousands of businesses")
Best for fast, reliable app email.
Postmark says thousands of businesses trust their email delivery to it but does not publish an exact number, so it ranks below vendors that do. It has been delivering email since 2010 and offers both an API and SMTP. Its standout feature is message streams: promotional email such as newsletters goes through separate infrastructure from transactional email, so a marketing blast cannot hurt delivery of password resets and receipts. Postmark vets new senders, includes premium support at no extra cost and has official libraries for many languages. Over the past 60 days, 95% of customers rated its support Great. Pricing starts at $15 a month for 10,000 emails, and a free developer plan lets you test with 100 emails a month. It fits SaaS teams that care most about fast, dependable delivery of app email.
Best forSaaS apps where password resets and receipts must arrive in seconds
Resend · Developer email API with React Email templates
No customer count published
The modern pick for React developers.
Resend does not publish a customer count, so it ranks below vendors that do. It calls itself the email API for developers and launched its sending platform in 2023 after starting as an open source project in 2022. Emails can be built as React components with the React Email library and sent through one API endpoint or official SDKs for Node.js, Python, PHP, Ruby, Go, Java, Laravel, Rust and .NET. Batch sending handles up to 100 messages per request, and webhooks report delivery events. It also supports inbound email, marketing broadcasts, a CLI and an MCP server for AI agents. The free plan includes 3,000 emails a month, limited to 100 a day, and Pro costs $20 a month for 50,000 emails. It suits developers who want a clean, modern API that fits a React codebase.
Best forDevelopers building with React and modern JavaScript frameworks
Skip it ifYou need an SMTP-first setup for legacy software
MailerSend · Transactional email, SMS and WhatsApp API with SMTP relay
No exact count published ("thousands of organizations")
Best for mixed technical and non-technical teams.
MailerSend says it is trusted by thousands of organizations but does not publish an exact number, so it ranks below vendors that do. It is built by the team behind MailerLite and was founded in 2020. It sends transactional email through a REST API or SMTP relay, adds toll-free SMS and WhatsApp messaging (WhatsApp in the USA and Canada only), and offers webhooks, real-time activity tracking, API and SMTP logs and automatic suppression lists. Code examples cover 8 languages and frameworks, including Node.js, PHP, Laravel, Python, Ruby, Go and Java. It promises 99.5% guaranteed uptime and 24/7 support. The free plan sends 500 emails a month and paid plans add higher monthly volumes (its site shows differing prices, so check the pricing page). It fits teams that want developers and non-technical staff to share the work.
Best forTeams where non-developers also edit templates
Twilio SendGrid is the largest, used by 12M+ developers and delivering 205+ billion emails a month through an Email API or SMTP relay. Brevo, with 600,000+ customers, is the best value for small businesses.
What is the cheapest transactional email service?
Amazon SES charges $0.10 per 1,000 emails on a pay-as-you-go basis. Among free plans, Brevo allows 300 emails a day (9,000 a month) and Mailjet up to 6,000 a month.
What is the difference between an email API and an SMTP relay?
An email API lets your code send email with an HTTP request and usually returns richer data and webhooks. An SMTP relay works with any software that already speaks SMTP, such as WordPress. Most services on this list, including SendGrid, Brevo, Mailgun and Postmark, offer both.
Which transactional email service is best for developers?
Resend is built for modern developers, with React Email templates and SDKs for Node.js, Python, PHP, Ruby, Go, Java, Laravel, Rust and .NET. Mailgun and Postmark are long-standing developer favorites.
How were these services ranked?
By the number of customers, businesses, developers or users each vendor publishes on its own website. These are the companies' own claims. Vendors that publish no exact count are ranked last.
How we ranked
Customers, businesses, developers or users each vendor says it serves, as published on its own website (read November 2026). Counts are each company's own claims and measure different things (SendGrid counts developers, Mailtrap counts monthly active users, the others count customers or businesses), so treat them as a guide to scale, not an audit. Amazon SES, Postmark, Resend and MailerSend publish no exact customer count and are ranked last, in no particular order of size. Every number on this page comes from the source linked under each pick, and every photo shows the actual product. Brands can't pay to be on this list.
Brevo600,000+ customers
Mailgun150,000+ businesses
