GAS.js

function sendEmailToServer() {
  
    var threads = GmailApp.search('is:unread');
    for(var i = 0; i < threads.length; i++){
    
      var messages = threads[i].getMessages();
      
      for(var j =0; j < messages.length; j++) 
      {
       
        var message = messages[j];
        if(message.isUnread()){
        
          var emailData = {
            subject: message.getSubject(),
            body:  message.getPlainBody()
          };
  
          var payload = JSON.stringify(emailData);
          var options = {
            'method': 'post',
            'contentType': 'application/json',
            'payload': payload
          }
          UrlFetchApp.fetch('https://d05e-38-75-137-97.ngrok-free.app/forward-email', options);
          message.markRead();
        }
      }
    }
}
  
  
function createTimeTrigger(){
    ScriptApp.newTrigger('sendEmail')
            .timeBased()
            .everyMinutes(5)
            .create();
}