title: Human Guide description: Human-maintained guidance for @lastshotlabs/slingshot-mail

Human-owned documentation. This package should stay explicit about provider, renderer, and queue boundaries.

Purpose

@lastshotlabs/slingshot-mail owns outbound mail delivery for Slingshot. It exists so packages can emit events and let mail delivery happen through a dedicated provider, renderer, and queue layer instead of embedding provider SDK calls throughout the codebase.

Design Constraints

• The package should remain route-free. Mail is a side-effect system, so it should activate in setupPost and listen to the event bus rather than pretending to be an HTTP feature package.
• Provider, renderer, and queue are separate contracts on purpose. They should not collapse into one "mail adapter" abstraction because transport, templating, and buffering evolve on different timelines.
• The plugin should stay safe to boot in small apps. When no queue is provided, it falls back to an in-memory queue instead of forcing infrastructure on every consumer.

Operational Notes

• createMailPlugin() validates provider and renderer shapes immediately and throws before startup if required methods are missing.
• Startup warnings about missing templates or failed provider health checks are diagnostics, not necessarily fatal errors. They usually mean the renderer inventory or external provider config needs attention.
• setupPost() should only run once for a given plugin instance. The package defends against double activation because queue startup and event subscriptions are stateful.
• The in-memory queue's drain() call has a default 30-second timeout. Configure it via drainTimeoutMs in the queue options (0 to disable). A timeout warning is logged if jobs are still in flight when the deadline is reached — the timeout is soft; jobs are not cancelled.

Gotchas

• The package does not export a shared ./testing entrypoint. Tests live in the package itself and are organized around provider, queue, and lifecycle behavior.
• from is required even when a provider has account-level defaults. Slingshot keeps the outgoing message shape explicit.
• Durable subscriptions are a transport decision, not just a boolean preference. If the event bus is in-process only, durable mode should not be documented as if it were guaranteed.

Key Files

• src/plugin.ts
• src/lib/subscriptionWiring.ts
• src/types/config.ts
• src/providers/*
• src/queues/*

Provider Setup Examples

Resend:

import {
  createMailPlugin,
  createRawHtmlRenderer,
  createResendProvider,
} from '@lastshotlabs/slingshot-mail';

const mail = createMailPlugin({
  provider: createResendProvider({ apiKey: process.env.RESEND_API_KEY! }),
  renderer: createRawHtmlRenderer({
    templates: {
      welcome: {
        subject: 'Welcome',
        html: '<p>Hello {{name}}</p>',
      },
    },
  }),
  from: '[email protected]',
});

SendGrid:

import { createSendgridProvider } from '@lastshotlabs/slingshot-mail';

const provider = createSendgridProvider({ apiKey: process.env.SENDGRID_API_KEY! });

AWS SES:

import { createSesProvider } from '@lastshotlabs/slingshot-mail';

const provider = createSesProvider({ region: 'us-east-1' });
// Credentials resolved via the AWS SDK credential chain (env, IAM, etc.)

Postmark:

import { createPostmarkProvider } from '@lastshotlabs/slingshot-mail';

const provider = createPostmarkProvider({ serverToken: process.env.POSTMARK_SERVER_TOKEN! });

All providers implement the same MailProvider contract — swap them without changing callers.