@Dino Giuditta Glad that you were able to resolve your issue and I appreciate your effort in sharing the solution.
Your contribution will undoubtedly assist others facing similar challenges.
As the Microsoft Q&A community follows a policy where the question author cannot accept their own answer , I've reposted your solution.
Feel free to consider "Accepting" the answer if you find it suitable.
Resolution- It's necessary to add below line of code before the "Send" method:
System.Net.ServicePointManager.SecurityProtocol = System.Net.SecurityProtocolType.Tls12;
Thanks again.