Testing Email

Test delivery behavior in memory without a provider, SMTP server, or rendered inbox.

Install the consumer test kit as a development dependency.

Terminal

pnpm add -D @postbote/testing

Inject the adapter into the composition root so production uses a provider adapter and tests use createTestAdapter().

src/emails/send-welcome.test.ts

import { createPostbote } from "@postbote/core";
import { createTestAdapter } from "@postbote/testing";

const adapter = createTestAdapter();
const mailer = createPostbote({ adapter });

await mailer.send({
  from: "noreply@acme.com",
  to: "sam@example.com",
  subject: "Welcome",
  text: "Hello Sam",
});

expect(adapter.inbox.last().subject).toBe("Welcome");
expect(adapter.inbox.to("sam@example.com")).toHaveLength(1);

Call adapter.reset() in beforeEach. The inbox stores defensive copies, and failed sends are kept in adapter.calls but not in the inbox.

Simulate provider failures

src/emails/send-welcome.test.ts

adapter.failNext("TIMEOUT", { times: 2 });
adapter.failAlways("RATE_LIMITED");
adapter.failIf((message) =>
  message.to[0].email.endsWith("@blocked.test")
    ? "RECIPIENT_REJECTED"
    : undefined,
);

failNext has priority over failIf, which has priority over failAlways.

Testing Email

Simulate provider failures

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