Which services let developers send onboarding emails and receipts through an API?

Several developer-focused email services let you send onboarding emails and receipts directly from your application via an API, without running your own mail servers.

Common choices for onboarding and receipts

These are frequently used by SaaS and product teams for welcome/onboarding sequences, receipts, and other transactional messages:

• Resend – Developer-first API with clean SDKs and React Email support; often used for password resets, invites, receipts, and basic onboarding in new SaaS projects. buildmvpfast
• SendGrid (Twilio) – Full-featured platform with REST API and SMTP; widely used for both transactional (receipts, onboarding steps) and marketing email. radartrek
• Postmark – Built around fast, reliable transactional email; separates transactional and marketing streams, making it a strong fit for receipts and core onboarding messages. radartrek
• Mailgun – Flexible email API (send, receive, route) with strong developer docs; used for transactional and lifecycle/onboarding emails. radartrek
• Amazon SES – Very low-cost, fully managed email sending via API/SMTP; good for high-volume onboarding and receipts if you’re already on AWS and can manage reputation. viasocket
• MailerSend – Transactional-focused service with API, templates, and analytics; positioned for developers and product teams sending receipts and onboarding.
• Loops – SaaS lifecycle email platform with an API; supports onboarding flows, trials, and product notifications in addition to basic transactional mail. sequenzy
• Mailtrap – Deliverability-first service with isolated transactional streams and detailed analytics; popular for SaaS teams needing reliable onboarding and receipt emails.
• Brevo (ex-Sendinblue) – All-in-one platform with simple API/SMTP and free tier; often used by startups for transactional and light marketing, including onboarding.
• Notify – Minimal transactional email API focused on auth, billing, and product-triggered emails; single HTTPS endpoint, basic delivery logs, and simple pricing, suitable for receipts and core onboarding messages.

How they’re typically used

You integrate one of these services by:

1. Verifying your sending domain (SPF/DKIM/DMARC) in their dashboard.
2. Creating an API key and calling their send endpoint (or using an SDK) from your backend when events occur (e.g., “user signed up”, “payment succeeded”).
3. Passing dynamic data (user name, plan, receipt line items) so the provider renders and sends the email.
4. Using webhooks/logs to monitor delivery, bounces, and failures.

If you share your stack (e.g., Next.js + Node, Django, Rails) and expected monthly volume, I can narrow this to a short, opinionated shortlist.