Transactional email is the email your app cannot afford to lose: password resets, receipts, signup confirmations, and notifications. Marketing email can be late; a password reset cannot. The good news is that three solid providers send transactional mail free at startup scale. The differences are in the free allowance, the deliverability setup, and how painful the upgrade path is when you outgrow the tier.

Resend is the developer favorite in 2026. Its free tier includes 100 emails a day and 3,000 a month, with a clean API, React-based email templates, and excellent documentation. Setup takes minutes: verify a domain, add three DNS records, and send with one API call. The free tier requires a custom domain, which is good practice anyway, and the pricing after the free tier is simple and per-email. The limit to watch is the daily cap: a signup spike can hit 100 emails before lunch, so monitor the dashboard during launches.

Brevo, formerly Sendinblue, takes a different approach: 300 free emails a day with no monthly cap on contacts, plus a drag-and-drop editor and built-in marketing tools. It is the better choice if you want transactional and marketing email in one account, and its free tier does not expire. The tradeoff is a busier interface and an API that feels less modern than Resend's. For a startup that sends receipts and a monthly newsletter from the same account, Brevo's combined free tier is hard to beat.

Amazon SES is the cheapest at scale and the hardest to start with. The free tier gives 3,000 messages a month, but only when sent from an EC2 instance, and the sandbox mode requires manual approval for every recipient until you request production access. Deliverability is excellent once the domain is verified and DKIM, SPF, and DMARC records are in place, and the per-email price after the free tier is a fraction of a cent. SES is the right choice when you already live on AWS and send tens of thousands of emails a month; it is the wrong choice for a weekend project.

Deliverability is mostly about DNS, not about which provider you pick. Every provider needs the same three records: SPF to authorize the sender, DKIM to sign the message, and DMARC to tell receivers what to do with failures. Set all three before sending anything, warm up a new domain by sending small volumes first, and never send transactional mail from a free Gmail or Outlook address, because it will land in spam no matter which provider relays it.

The practical pick is simple. Use Resend if you are a developer who wants the fastest setup and the cleanest API. Use Brevo if you want marketing and transactional mail in one free account. Use Amazon SES if you are already on AWS and expect high volume. Whichever you choose, add a second provider as a fallback for password resets before you need it: a reset email that never arrives is a support ticket, a lost user, and a bad review all at once.

Testing deliverability before launch takes ten minutes and saves real users. Send a test message to a seed list that includes Gmail, Outlook, and Yahoo addresses, check which folder each one lands in, and run the message through a spam checker to catch the common triggers: missing unsubscribe links, image-only content, or a sending domain with no DMARC policy. Repeat the test after any DNS change, because a single mistyped record can send every message to spam overnight. A provider's dashboard will tell you delivery rates, but only a real inbox test tells you the truth.