We have analyzed your shared information regard the exception raised by the SmtpClient and found that there are multiple instances of configuration elements your application. Please correct these and let us know if you still face any such issue? In case the issue still exists, please share your sample application with us that we can use to reproduce the problem at our end.
Please check the attached project. I have removed the DLLs to reduce its size. Also, the smtp client log file is also empty. This happens with all the projects created on this specific PC. If I use Exchange client, it sends out emails without any issues.
The Exception details shared by you show that there are some firewall settings that are stopping the SmtpClient to send out email. Please check your firewall settings and let us know your feedback.