How can I put my email into R2

Hello, I want put my email into R2. Now I convert message.raw stream to ArrayBuffer, then save ArrayBuffer to R2.

async function streamToArrayBuffer(stream, streamSize) {
  let result = new Uint8Array(streamSize);
  let bytesRead = 0;
  const reader = stream.getReader();
  while (true) {
    const { done, value } = await reader.read();
    if (done) {
      break;
    }
    result.set(value, bytesRead);
    bytesRead += value.length;
  }
  return result;
}
export default {
  async email(message, env, ctx) {
    console.log(message.to);
    const rawEmail = await streamToArrayBuffer(message.raw, message.rawSize);
    await env.r2.put(`${message.to}/${message.from}-${new Date().getTime()}.eml`, rawEmail,{customMetadata:{
      from:message.from,
      time:new Date().getTime(),
    }});
  }
}

It cost more CPU time instreamToArrayBuffer function, I think I can do it like await env.MY_BUCKET.put(key, request.body);.
But when I put message.raw into R2 directly, I got TypeError: Provided readable stream must have a known length (request/response body or readable half of FixedLengthStream) . How can I fix it?thanks