📧 Email Generator Microservice

A lightweight, self-hostable service for rendering HTML emails with React Email.

• Hassle-free & maintenance-free – runs entirely on the free tier of Cloudflare Workers.
• Simple to use – perfect for login codes, registration flows, and other transactional emails.
• Flexible – develop locally with a Bun server, then deploy serverlessly with zero configuration.
• Clean separation – does not pollute your project with react or react-email imports or dependencies.

Whether you need a quick drop-in solution or a fully open-source foundation for your project, this microservice makes email generation easy and reliable.

Quick Links

• code - https://github.com/david1gp/email-generator
• npm - https://www.npmjs.com/package/@adaptive-ds/email-generator
• react email docs - https://react.email/docs/getting-started/manual-setup

Features

• Renders HTML and plain text email templates.
• Supports internationalization (English and German).
• Validates input using Valibot schemas.
• Includes server timing headers for performance monitoring.
• Endpoints: /renderEmailTemplate/signUpV1, /renderEmailTemplate/signInV1, /renderEmailTemplate/orgInvitationV1, /renderEmailTemplate/teamInvitationV1, /renderEmailTemplate/invoiceV1, and /renderEmailTemplate/markdownV1.

Pre-Send HTML Size Budget (90 KiB)

• Origin & Purpose: All supported project email templates enforce a 90 KiB (92,160 bytes) pre-send HTML size budget. This acts as a pre-send HTML guard to keep rendered emails comfortably below Gmail's observed 102 KiB (102,400 bytes) message clipping threshold ([Message clipped] View entire message).
• Scope & Coverage: The budget covers the raw pre-send HTML output rendered by the service for every supported email template (signUpV1, signInV1, invitationV1, orgInvitationV1, teamInvitationV1, passwordChangeV1, emailChangeV1, invoiceV1, and markdownV1 for supported input profiles).
• Caveats & MIME Growth:
  • The 90 KiB limit is a pre-send HTML guard measured on raw service output prior to ESP delivery.
  • Final ESP-delivered MIME messages can grow in size during transit due to MIME transfer encoding (such as quoted-printable or base64), tracking wrappers/pixels added by Email Service Providers, custom headers, boundary markers, or attached plain-text/multipart representations.
  • For markdownV1, the 1,000-character input limit keeps approved two-column tables, bullet lists, key/value transposes, and paragraph content under 90 KiB. Wide, out-of-contract multi-column stress tables (such as dense 4-column pipe tables filling 1,000 characters) are not supported and can exceed 90 KiB.

Trusted Markdown Template (markdownV1)

The /renderEmailTemplate/markdownV1 endpoint renders trusted, self-authored Markdown content into standard HTML and plain text email formats.

Security & Authentication

• Requires Authorization: Bearer <MARKDOWN_RENDER_TOKEN> header.
• Environment variable: MARKDOWN_RENDER_TOKEN. Must be configured on the server/worker. Requests fail closed (401 Unauthorized) when absent or invalid.
• Body size limit: 32,768 bytes (413 Payload Too Large if exceeded). markdown field limit: 1,000 characters so all supported input profiles (including approved two-column tables, key/value transposes, and bullet lists) remain below the 90 KiB pre-send HTML budget. Note that dense out-of-contract four-column stress tables filling 1,000 characters can exceed 90 KiB pre-send HTML and are not supported.
• Trusted Author Model: Designed strictly for self-authored/system content delivered through authenticated private service boundaries. It does not sanitize raw HTML or validate arbitrary URL schemes. If user-generated, CMS, or untrusted content parsing is required in the future, migrate to an AST/component-based renderer.

Approved Syntax & Table Guidelines

• Supported syntax: Paragraphs, bold text (**bold**), absolute URLs / labeled links ([label](https://...)), unordered bullet lists (- item), and simple pipe tables.
• Table limits: At most 2 columns, one header row, short breakable text/labels, simple pipe syntax. Labeled links are preferred over bare URLs inside table cells.
• Wider records (3+ fields): Format as bullet lists (e.g., - **Label:** Value 1; Value 2; [Link](https://...)) or a two-column Field / Value transpose table, or link to an external web table.
• Headings, ordered lists, italics, strikethrough, block quotes, code blocks, images, task controls, raw HTML, and SVGs are unsupported by contract.

Environment Variables

Variable	Required	Description
MARKDOWN_RENDER_TOKEN	Yes (for markdownV1)	Bearer token secret required to access the trusted markdownV1 rendering route. Set via Worker secrets or environment variables.

Templates

signUpV1 - Sign-up / Registration	signInV1 - Sign-in / Login / Forgot password
passwordChangeV1 - Change/Reset Password	emailChangeV1 - Change Email
orgInvitationV1 - Organization invitation	teamInvitationV1 - Team invitation
invoiceV1 - Invoice (paid / unpaid variants)	markdownV1 - Trusted Markdown

Prerequisites

• Node.js (for package management) or Bun.
• Cloudflare account (for Workers deployment).

Local Development

1. Clone the repository.
2. Install dependencies:

   bun install
   

With Bun Server

• Start the development server:

  bun run start
  

  The server runs on http://localhost:3055 (port configurable via src/server/serverPortBun.ts).

• For React Email preview (optional):

  bun run dev:email
  

  This starts the React Email preview server for template development.

With Cloudflare Workers

• Start the local Worker development server:

  bun run dev:worker
  

  The Worker runs on http://localhost:8787 (default Wrangler port).

• To test endpoints, send POST requests to:

  • http://localhost:8787/renderEmailTemplate/signUpV1

Testing

Run tests with Bun:

bun run test

Or in watch mode:

bun run test:w

Tests cover API rendering for login codes (extend for registration as needed).

Deployment to Cloudflare Workers

1. Login to Cloudflare:

   wrangler login
   

2. Configure Account ID (if needed, add to wrangler.toml):

   wrangler whoami
   

   Then update wrangler.toml with account_id = "your-account-id".

3. Deploy:

   bun run deploy
   

   (Or npx wrangler deploy.)

4. Monitor:

   wrangler tail email-generator-worker