Build the raw message content for the forwarded email

PrepareMessage.ts

// Build the raw message content for the forwarded email
let rawContent = `Content-Type: message/rfc822\n` +
                 `Content-Disposition: attachment; filename="original.eml"\n` +
                 `Content-Transfer-Encoding: base64\n\n` +
                 `${res.data.raw}`;

// Iterate over the original message's attachments and add them to the forwarded email
res.data.payload.parts.forEach(part => {
  if (part.body.attachmentId) {
    rawContent += `\nContent-Type: ${part.mimeType}; name="${part.filename}"\n` +
                  `Content-Disposition: attachment; filename="${part.filename}"\n` +
                  `Content-Transfer-Encoding: base64\n\n` +
                  `${part.body.data}`;
  }
});

// Create a new message that includes the original message and its attachments as attachments
gmail.users.messages.send({
  userId: 'me',
  requestBody: {
    to: recipient,
    subject: subject,
    message: {
      raw: Buffer.from(rawContent).toString('base64')
    }
  }
}, (err, res) => {
  if (err) {
    console.error(err);
    return;
  }
  console.log(`Message forwarded to ${recipient} with ID: ${res.data.id}`);
});