Using .net 4.7.1 configuration builders to configuration SmtpClient from AppSettings - https://github.com/aspnet/MicrosoftConfigurationBuilders

ISmtpMailDeliverySettings.cs

    public interface ISmtpMailDeliverySettings
    {
        string Host { get; }
        int Port { get; }
        string Username { get; }
        string Password { get; }
        bool EnableSSL { get; }
    }

SmtpMailDeliverySettings.cs

    public class SmtpMailDeliverySettings : ISmtpMailDeliverySettings
    {
        private const string Prefix = "Smtp:";
        private readonly NameValueCollection nvc;

        public SmtpMailDeliverySettings() : this(ConfigurationManager.AppSettings)
        {
        }

        public SmtpMailDeliverySettings(NameValueCollection nvc)
        {
            this.nvc = nvc ?? throw new ArgumentNullException(nameof(nvc), "You must supply a NameValueCollection");
        }

        private int ParseInt(string key, int defaultValue)
        {
            if (int.TryParse(nvc[key], out var value))
            {
                return value;
            }

            return defaultValue;
        }

        private bool ParseBoolean(string key, bool defaultValue)
        {
            if (bool.TryParse(nvc[key], out var value))
            {
                return value;
            }

            return defaultValue;
        }

        private string ParseString(string key, string defaultValue = "")
        {
            var value = nvc[key];
            if (!string.IsNullOrWhiteSpace(value))
            {
                return value;
            }

            return defaultValue;
        }

        public string From => ParseString($"{Prefix}{nameof(From)}");

        public string Host => ParseString($"{Prefix}{nameof(Host)}");

        public int Port => ParseInt($"{Prefix}{nameof(Port)}", 25);
        
        public string Username => ParseString($"{Prefix}{nameof(Username)}");

        public string Password => ParseString($"{Prefix}{nameof(Password)}");

        public bool EnableSSL => ParseBoolean($"{Prefix}{nameof(EnableSSL)}", false);
    }

SmtpMailSettingsConfigBuilder.cs

    public class SmtpMailSettingsConfigBuilder : ConfigurationBuilder
    {
        public SmtpMailSettingsConfigBuilder() : this(ConfigurationManager.AppSettings)
        {

        }

        public SmtpMailSettingsConfigBuilder(NameValueCollection nvc) : this(new SmtpMailDeliverySettings(nvc))
        {

        }

        public SmtpMailSettingsConfigBuilder(ISmtpMailDeliverySettings smtpMailDeliverySettings)
        {
            
        }

        public override ConfigurationSection ProcessConfigurationSection(ConfigurationSection configSection)
        {
            if (configSection is SmtpSection smtpSection)
            {
                var config = new SmtpMailDeliverySettings();
                smtpSection.From = config.From;
                smtpSection.DeliveryMethod = SmtpDeliveryMethod.Network;
                smtpSection.Network.Host = config.Host;
                smtpSection.Network.Port = config.Port;
                smtpSection.Network.UserName = config.Username;
                smtpSection.Network.Password = config.Password;
                smtpSection.Network.EnableSsl = config.EnableSSL;

                return smtpSection;
            }
            else
            {
                return base.ProcessConfigurationSection(configSection);
            }
        }
    }

Usage.cs

public async Task SendEmail()
{
    using (var smtpClient = new SmtpClient())
    {
        await smtpClient.SendMailAsync("phil@localtest.me", "phil@localtest.me", "Test", "testing email config");
    }
}

web.config

<configuration>
  <configSections>
    <section name="configBuilders" type="System.Configuration.ConfigurationBuildersSection, System.Configuration, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" restartOnExternalChanges="false" requirePermission="false" />
  </configSections>
  <configBuilders>
    <builders>
      <add name="MailSettingsConfigurationProvider" type="<Namespace-Path-To-Class>.SmtpMailSettingsConfigBuilder, <assembly-name>" />
    </builders>
  </configBuilders>
  <appSettings>
    <add key="Smtp:From" value="phil@localtest.me" />
    <add key="Smtp:Host" value="mail.localtest.me" />
    <add key="Smtp:Port" value="25" />
    <add key="Smtp:Username" value="username" />
    <add key="Smtp:Password" value="supersecret" />
  </appSettings>
  <system.net>
    <mailSettings>
      <smtp configBuilders="MailSettingsConfigurationProvider" />
    </mailSettings>
  </system.net>
</configuration>