You should update your application to the latest .net framework version (e.g. .net 4.8 or at least .net 4.5) and update to the most recent Mail.dll email component. Mail.dll library supports latest .net versions: .net 6, .net 7 as well as latest .net framework version 4.8.
You can then use
smtp.SSLConfiguration.EnabledSslProtocols = SslProtocols.Tls12
It is possible to use TLS 1.2 in applications targeting earlier .NET framework versions (2.0 - 4.0), however .net framework 4.5 must be installed on the running machine.
Your old app will use the 4.5 System.dll and you can enable TLS 1.2 using the trick with 3072 value.