index.js

const { google } = require('googleapis');
exports.sendEmail = async(req,res)=>{
var body = req.body[0]
  var to = body.to[0];
  var arrayBody = to.replace('[','');
  arrayBody = arrayBody.replace(']','');
  arrayBody = arrayBody.split(',');
  var entryArray = []
  arrayBody.forEach(elm =>{
    if(elm.indexOf(']') >-1){
      elm = elm.replace(']','')
    }
    entryArray.push((JSON.parse(elm)).email)
  })
  
  const oAuth2Client = new google.auth.OAuth2(
    process.env.CLIENT_ID,
    process.env.CLIENT_SECRET,
    process.env.REDIRECT_URI
  );
  const auth = await oAuth2Client.setCredentials({ refresh_token: process.env.REFRESH_TOKEN});
  var gmail = google.gmail({version : 'v1', auth : oAuth2Client})

  if(entryArray.length >24999){
    //split up into smaller tasks and time out for another task in a few minutes to avoid passing the Gmail quota limits
  }
  else{
     //begin email body construction. Feel free to change out the subject directly in the code or by sending a different "Subject" request body parameter from the source (Integromat, Zapier, Etc.)
     const subject = body.Subject;
     const utf8Subject = `=?utf-8?B?${Buffer.from(subject).toString('base64')}?=`;
    
     //send to every email in the list  
    for(i=0;i<entryArray.length;i++){
      try{
        const messageParts = [
          'From: ' + process.env.FROM_EMAIL,
          'To: ' + entryArray[i],
          'Content-Type: text/html; charset=utf-8',
          'MIME-Version: 1.0',
          `Subject: ${utf8Subject}`,
          '',
          body.Body //this is assuming you're sending some html code as a string from your API (Integromat, Zapier Etc). If you want to just write the HTML code directly in here you can do so by wrapping the entire html code as a string
        ];
        const message = messageParts.join('\n');
      
        // The body needs to be base64url encoded.
        const encodedMessage = Buffer.from(message)
          .toString('base64')
          .replace(/\+/g, '-')
          .replace(/\//g, '_')
          .replace(/=+$/, '');

      const response = await gmail.users.messages.send({
        userId: 'me',
        requestBody: {
          raw: encodedMessage,
        },
      });
      console.log(response.data);
      }
      catch(err){
        console.log(err);
      }
    }
  }
  
  
  res.sendStatus(200)
}