Building a Transactional Email System with Next.js and PostaSend

Transactional email — the emails your application sends in response to user actions — is one of those engineering tasks that looks simple until you have to own the deliverability. Password resets that end up in spam folders, invoice emails that bounce off corporate mail servers, welcome emails that trigger unsubscribe floods: these are real support tickets waiting to happen. A well-integrated email platform takes the guesswork out of it.

This guide walks through a complete integration in a Next.js 15 App Router project. We will build three email flows: account creation confirmation, password reset, and invoice delivery with a PDF attachment. We will use PostaSend's TypeScript SDK, which ships with full type definitions and works natively in the Next.js runtime. The pattern is consistent across all three flows: trigger from a Server Action or Route Handler, call client.emails.send(), store the returned messageId against the relevant database record for audit purposes.

One architectural decision worth calling out: where to put the PostaSend client initialization. Because the SDK client is stateless and safe to share across requests, initialize it once in a lib/postasend.ts module and import it wherever you need it. Avoid initializing it inside Server Actions or Route Handlers — it adds unnecessary allocation overhead on every request and makes it harder to mock in tests. The client is safe for module-level singleton use.

For local development, use PostaSend's test mode by setting POSTASEND_MODE=test in your .env.local file. In test mode, the API accepts all requests and returns realistic responses, but emails are not actually delivered — they appear in your PostaSend dashboard under Test Sends. This lets you exercise your integration code without sending real emails during development. For end-to-end testing, PostaSend supports a --dry-run flag on the send call that validates your payload against the API schema without queuing the email.