<PackageReference Include="NETStandard.Library" Version="2.0.0-preview2-25401-01" />

System.Net.Mail.SmtpDeliveryMethod

public enum SmtpDeliveryMethod
namespace System.Net.Mail { public enum SmtpDeliveryMethod { Network = 0, PickupDirectoryFromIis = 2, SpecifiedPickupDirectory = 1 } }