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[System.Obsolete("SmtpClient and its network of types are poorly designed, we strongly recommend you use https://github.com/jstedfast/MailKit and https://github.com/jstedfast/MimeKit instead")]
public class SmtpClient : IDisposable
I've been through that too and switched to Mailkit and Mimekit using Nuget to take care of the dependencies.
I think you may find that if you change the SMTP Port you should still be able to use the .NET SmtpClient.
It should be port 25 for ssl connections that still works with the old .NET class - I have some unit tests with my old .NET SmtpClient class using port 25 and the tests pass.
“That which can be asserted without evidence, can be dismissed without evidence.”
Of course only time can show "usefullness" of the class and it's normal when you find "obsolete" of design which was done 10 years ago. BUT... why MS itself not fixing anything?? WHAT they are busy that they cannot rewrite one of most important classes? (be sure, mail notifications used everywhere!)
MS can just make show how cool they are, "hard working", "hiring the best" and other BS. But reality shows they hire 10 indians for $100/month and do WORST code in the world. How long MS suppose to roll down??