Email Testing Guide for Developers (2026 Edition)

Email is the test surface nobody wants to own. It's asynchronous, depends on external providers, has its own version of "flaky" (sometimes a delivery just… takes 90 seconds), and the bugs that bite hardest in production — wrong template variables, broken unsubscribe links, OTPs that expire too fast — are the ones unit tests can't catch. This guide is the working developer's playbook for testing email in 2026. We'll cover the three layers (unit, integration, end-to-end), the tooling for each, an...