Hi @OUEDRAOGO Issouf [NSIA Banque Côte d'Ivoire] , this error indicates that there's an issue with the recipient list.
Please check the recipient list in the request body of the email message. Make sure that the recipient list is properly formatted and that all email addresses are valid.
You can also check if the email addresses of your students are added to the allowed email sender domains under the Email Communication Services resource. The domains added under this resource type will contain all the sender authentication and engagement tracking configurations that are required to be completed before start sending emails. Once the sender domain is configured and verified, you'll able to link these domains with your Azure Communication Services resource and you can select which of the verified domains is suitable for your application and connect them to send emails from your application.
Please let me know if you have any questions and I can help you further.
Best,
James