title: Get notified when Email content invalid or mail is not delievered in php using sendgrid API
body: <p>We are sending about 1 lac email daily using sendgrid API.</p>
  
  <p>But what happen is thousand of mail are undelievered to the recipient.</p>
  
  <p>we want to get the list of these undelievered mail. How is that possible ?</p>
  
  <p>Please suggest some idea or links. becouse I am lost and would really appreciate the assistance. </p>
  
  <p>Thank you.</p>