If you are experiencing an issues with your emails getting rejected or bounced back you will usually receive an email notifying you of this action. Usually the reason is stated in the notification email like the one below, 



The three most common causes for this issue are as follows


  1. You have attached a file that exceeds the email file size limit ( 25 MB is the limit) Try to attach less files or use OneDrive to share out these larger files
  2. Your cached contacts are corrupted so you receive a message that the contact does not exist. This can be resolved by clearing out the contact and reentering the email. You can do this by following the instructions here 
  3. You are sending a file that is normally reject by the persons organization. Below is a list of the most common files that are not allowed in emails.