Thank you for the detailed response.
You are right about the outbound adapter but in reality the code I have provided was a test code I have developed because the adapter did not work.
Very probably both the adapter and the %Net.SMTP class do not work in my case because of configuration problems with outlook, since I have obtained successfull results using gmail instead. 
Regarding the user/password or OAuth2, I was provided with code of old projects that used the same configuration and it worked just fine, while now that I am working with a new project it does not anymore.
I am sure the version of IRIS is 2021.1, I have just checked in the portal.
I will try different settings based on your response

The configuration was already present in the production, but still I get the error that the property UseSTARTTLS is not available.
I have switched to use gmail for the sender instead of Outlook following this guide
Configuring email alerts with Gmail on IRIS management portal |
and it works just fine, but I wonder what is the problem with the configuration settings to use Outlook.

Thank you for the response.
It appears the t and p reversed was just a typo error, in the production I had set the correct names.
If I try to set the secure property of the server to STARTTLS I obtain an error related to the property does not exist, this is why I had commented it in the code aboce