@HAMZA EL FALLOUSSI Thanks for posting your question in Microsoft Q&A, apologize for any inconvenience caused on this.
Based on shared information, I have understood that you are trying to setup SMTP authentication and sending the email through ACS resource using Turbo mailer 3rd party application and it is failing.
I hope you have followed these documentation steps, to setup the SMTP authentication.
- Have tried sending the email through PowerShell or through C# SDK as mentioned in the documentation here and were you able to send the email successfully?
- Also, have you tried sending the email with any other 3rd party application rather than this turbo mailer? and facing the same issue.