There are three possible explanations for an exclamation mark appearing in the transmission of your message: 

  1. There was an error in the construction of the recipient's email. Simply click on the exclamation mark icon to correct the recipient's email address and resend. 
  2. The recipient has put the notification directly into their spam folder without opening it or the notification has automatically gone into their spam folder.

    According to the laws in force, if your recipient has reported you as Spam, you will no longer be able to communicate with this recipient. You will have to find another way to communicate with your recipient.
  3. Your recipient's mailbox is full and therefore cannot receive your notification.

As you will have understood, these three causes are totally out of our control.