Flag: "React is not defined" in production builds with tsup + emitDecoratorMetadata

[sfbaker7]

What versions are you using? (if relevant)

@react-email/components@1.0.11, react-email@5.2.10, etc.

Describe the Bug

Not a react-email bug, just a gotcha that might be worth a docs mention since it affects an unrelated tsconfig flag.

What I encountered:
(tsup bug)
With emitDecoratorMetadata: true, tsup silently routes all .tsx files through SWC instead of esbuild. SWC defaults to React.createElement, ignoring both your tsconfig's jsx: "react-jsx" and any esbuildOptions you set. Combined with external: ["react"] (per react-email's recommendation), you get an error like:

ReferenceError: React is not defined
    at MyEmail (file:///app/backend/dist/index.js:28090:3)
    at sendMyEmail (file:///app/backend/dist/index.js:28548:36)
    at process.processTicksAndRejections (node:internal/process/task_queues:103:5)
    at async emailResolver.sendEmail 
(file:///app/backend/dist/index.js:28371:5)

Who's affected

Anyone using react-email with tsup whose tsconfig includes emitDecoratorMetadata: true (E.g. if you use TypeORM, type-graphql, NestJS)

What is affected (leave empty if unsure)

No response

Link to the code that reproduces this issue

N/A

To Reproduce

tsconfig.json:

{
  "compilerOptions": {
    "emitDecoratorMetadata": true,
    "experimentalDecorators": true,
    "jsx": "react-jsx"
  }
}

tsup.config.ts:

import { defineConfig } from "tsup";

export default defineConfig({
  entry: ["src/index.ts"],
  format: "esm",
  external: ["react"],
  esbuildOptions(options) {
    options.jsx = "automatic";
    options.jsxImportSource = "react";
  },
});

emails/Foo.tsx:

import { Container, Text } from "@react-email/components";

export default function FooEmail({ name }: { name: string }) {
  return (
    <Container>
      <Text>Hello {name}</Text>
    </Container>
  );

Expected Behavior

Bundle output contains React.createElement() with no React import.

Workarounds

export default defineConfig({
  // ...
  swc: {
    jsc: {
      transform: {
        react: {
          runtime: "automatic",
          importSource: "react",
        },
      },
    },
  },
});

What's your node version? (if relevant)

No response