# Stampwing > Email for your websites and apps Free, public developer resources about transactional email. The canonical articles include assumptions, primary sources, author attribution, and review dates. Markdown versions contain the same article content and section links. Availability: the public website and resources are live. Customer accounts, live sending, and the hosted MCP integration for Claude, ChatGPT, and Codex are not available yet. Architecture previews are not live-service documentation. ## Developer guides - [How a transactional email API works](https://www.stampwing.com/guides/transactional-email-api): Learn how a transactional email API handles authentication, queues, retries, and delivery events. Try a local cURL example that sends no real email. - [Markdown version](https://www.stampwing.com/guides/transactional-email-api/markdown) - [Email says “delivered” but never arrived: a developer’s guide](https://www.stampwing.com/guides/email-delivered-but-not-received): Trace a missing transactional email from API request to receiving server. A practical checklist for queues, bounces, spam, quarantine, and uncertain sends. - [Markdown version](https://www.stampwing.com/guides/email-delivered-but-not-received/markdown) - [How to manage transactional email across multiple apps and domains](https://www.stampwing.com/guides/transactional-email-multiple-projects): Design a manageable email setup for multiple SaaS apps: project-specific keys, sending domains, Test and Live environments, webhooks, limits, and ownership. - [Markdown version](https://www.stampwing.com/guides/transactional-email-multiple-projects/markdown) - [How to prevent duplicate emails when an API request times out](https://www.stampwing.com/guides/prevent-duplicate-emails): Use durable business events and idempotency keys to recover from email API timeouts. Includes a runnable no-send lab for lost responses and conflicting retries. - [Markdown version](https://www.stampwing.com/guides/prevent-duplicate-emails/markdown) - [Password reset email templates: HTML, plain text, and implementation notes](https://www.stampwing.com/guides/password-reset-email-templates): Download a free password reset email in HTML and plain text. Includes subject lines, fallback links, expiry copy, accessibility notes, and implementation checks. - [Markdown version](https://www.stampwing.com/guides/password-reset-email-templates/markdown) - [How to test transactional email without sending to real users](https://www.stampwing.com/guides/test-transactional-email): A practical testing strategy for email templates, API calls, retries, and webhooks. Includes a local mock server and clear limits on what simulation proves. - [Markdown version](https://www.stampwing.com/guides/test-transactional-email/markdown) - [SPF, DKIM, and DMARC for app developers: setup and troubleshooting](https://www.stampwing.com/guides/spf-dkim-dmarc-for-developers): Understand sending domains, SPF, DKIM selectors, and DMARC alignment with DNS commands, examples, and a practical troubleshooting sequence. - [Markdown version](https://www.stampwing.com/guides/spf-dkim-dmarc-for-developers/markdown) - [Send transactional email with Next.js: from request to delivery status](https://www.stampwing.com/guides/send-email-nextjs): Build a Next.js App Router email flow with a safe local fixture, stable request keys, server-side calls, error handling, and a message status lookup. - [Markdown version](https://www.stampwing.com/guides/send-email-nextjs/markdown) - [Email webhooks that survive retries and out-of-order events](https://www.stampwing.com/guides/reliable-email-webhooks): Verify signed email webhooks, commit events before acknowledging them, deduplicate retries, and preserve out-of-order observations. Includes a receiver and replay fixture. - [Markdown version](https://www.stampwing.com/guides/reliable-email-webhooks/markdown) ## Free tools - [SMTP status code lookup](https://www.stampwing.com/tools/smtp-status-code-lookup): Look up SMTP errors like 550, 421, and 5.1.1. Understand temporary and permanent failures, with practical next steps and links to the standards. - [Email send-time calculator](https://www.stampwing.com/tools/email-send-time-calculator): Estimate how long an email batch takes to submit from your recipient count, sending rate, and existing queue. Share the result and its assumptions. - [Email retry and backoff calculator](https://www.stampwing.com/tools/email-retry-calculator): Plan capped exponential backoff for email API or webhook retries. Compare fixed delays with full jitter and share a complete retry schedule. - [Email header analyzer](https://www.stampwing.com/tools/email-header-analyzer): Browser-only analysis of reported authentication results. ## Free email templates - [Password reset email](https://www.stampwing.com/templates/password-reset): HTML and plain-text downloads, variables, sample preview, and implementation checks. - [Email verification email](https://www.stampwing.com/templates/email-verification): HTML and plain-text downloads, variables, sample preview, and implementation checks. - [Payment receipt email](https://www.stampwing.com/templates/payment-receipt): HTML and plain-text downloads, variables, sample preview, and implementation checks. - [Account notification email](https://www.stampwing.com/templates/account-notification): HTML and plain-text downloads, variables, sample preview, and implementation checks. ## About - [Security architecture preview](https://www.stampwing.com/#security): Implemented application controls and their launch status. - [Editorial policy](https://www.stampwing.com/resources/editorial): How the resources are written and reviewed. - [Sitemap](https://www.stampwing.com/sitemap.xml): Canonical public pages. Scope: email acceptance is not proof of inbox placement. Examples use synthetic data; calculators do not send mail. Template previews do not establish compatibility with every email client.