When you have .config file:

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
  <appSettings>
    <add key="MailNotificationSettings.SmtpServerAddress" value="http://localhost"/>
    <add key="MailNotificationSettings.Username" value="user123"/>
    <add key="MailNotificationSettings.Password" value="passwd123"/>
  </appSettings>
</configuration>

Define the following interface:

public interface IMailNotificationSettings
{
    Uri SmtpServerAddress { get; }
    string Username { get; }
    string Password { get; }
}

And from your program use this code:

public static void Main()
{
        ConfigurationReader configReader = new ConfigurationReader().SetupConfigOf<IMailNotificationSettings>();
        IMailNotificationSettings mailNotificationSettings = configReader.ConfigBrowser.Get<IMailNotificationSettings>();
        
        var mailNotificationService = new MailNotificationService(mailNotificationSettings);
        
        //
        //      Do the work..
        //
}