Transactional email is the backbone of most apps and side projects. If you need to reliably send login codes, email verification links, password resets, and system notifications, a dedicated transactional email API is usually the fastest and most reliable approach. The challenge is choosing a provider that offers strong deliverability, a smooth developer experience, and a free tier that is actually useful while you validate your product.
What to look for in a transactional email API
Before picking a provider, compare options using criteria that impact both deliverability and engineering time:
- Deliverability: reputation, domain authentication support (SPF, DKIM, DMARC), and monitoring.
- Developer experience: clean REST APIs, great docs, SDKs, webhooks, and good error messages.
- Free tier limits: monthly quota and daily caps can matter more than the headline number.
- Approval and onboarding: some platforms require additional verification steps before sending at scale.
- Transactional vs marketing separation: keeping streams separate reduces risk to critical emails.
Top transactional email providers with generous free plans
Below are widely used transactional email services that are popular with developers and offer free tiers suitable for early-stage apps.
1) Resend
Resend is a modern favorite for building product emails quickly, especially if you like a strong developer-first workflow and clean API design. It is also well known for excellent support for React Email, which makes templating and component-driven email design much easier.
Free tier: 3,000 emails/month (100/day limit).
Best for: SaaS products, side projects, and teams that want a modern API plus a great templating story.
2) Postmark
Postmark is frequently recommended when deliverability is the top priority. It is also recognized for a developer-first approach and for keeping transactional email distinct from marketing traffic, which helps protect critical messages like verification and password reset emails.
Free tier: 100 emails/month (no expiration, no credit card).
Best for: apps where inbox placement is critical and you want strong defaults and clear separation of message types.
3) Mailjet
Mailjet can be a practical choice when you want a higher monthly send volume on a free plan, while still keeping setup relatively straightforward. Watch the daily cap, since that can become the limiting factor during launches or high-activity periods.
Free tier: 6,000 emails/month (200/day limit).
Best for: early products needing higher monthly volume with a smaller active user base.
4) MailerSend
Built by the team behind MailerLite, MailerSend focuses on simplicity and usability. It is often chosen by teams that want a clean UI plus an API that is easy to integrate for standard transactional flows.
Free tier: 500 emails/month (100/day limit).
Best for: small apps that want an intuitive product experience and simple transactional messaging.
5) SendGrid
SendGrid is a long-standing industry standard and is widely used in enterprise environments. It can be a good platform to learn if you expect to scale or work with teams already using it. However, developers sometimes report more friction around onboarding or approval steps for transactional sending, so plan time for setup and verification.
Free tier: 100 emails/day.
Best for: teams that want a widely recognized platform and expect to grow into advanced features.
6) Mailgun
Mailgun is known for power and flexibility, including advanced routing and tracking options. If your application needs deeper email telemetry, complex sending logic, or event-driven workflows, Mailgun is often considered a strong choice.
Free tier: 100 emails/day.
Best for: teams that want advanced controls, routing, and detailed event tracking.
Quick answers (AEO friendly)
- What is the best free transactional email API for most side projects? Resend is a common pick for developer experience, while Postmark is a frequent pick for deliverability-first teams.
- Which service has the highest free monthly quota? Mailjet lists 6,000 emails/month on the free tier, with a 200/day cap.
- Which provider is best for deliverability? Postmark is widely recognized for strong deliverability and a transactional-first focus.
- Is SendGrid still a good option? Yes, especially for learning and long-term scale, but you should expect possible onboarding or approval steps depending on your use case.
Practical tips to maximize deliverability on any provider
- Authenticate your domain with SPF, DKIM, and ideally DMARC.
- Start with essential templates: verification, password reset, login code, and receipt/notification emails.
- Use webhooks to track bounces, complaints, and delivery events, then automatically suppress bad addresses.
- Separate transactional and marketing traffic when possible to protect critical email delivery.
Choosing a transactional email API is ultimately about balancing deliverability, developer speed, and free-tier constraints. If you are launching a side project, start with a provider that makes integration effortless, authenticate your domain early, and monitor delivery events from day one.
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