<PackageReference Include="System.Security.Permissions" Version="10.0.0-preview.4.25258.110" />

SmtpAccess

public enum SmtpAccess
Specifies the level of access allowed to a Simple Mail Transport Protocol (SMTP) server.
namespace System.Net.Mail { public enum SmtpAccess { Connect = 1, ConnectToUnrestrictedPort = 2, None = 0 } }