How to use "System.Net.Mail.dll" When "Api Compatibility Level" is Net 2.0 Subset?

I tried to put the Net.Mail.dll in project , but it error with “Editor can only use assemblies targeting .Net 3.5 or lower”…
Or is there a way to add a desired dll when the “Api Compatibility Level” is .Net 2.0 Subset ?

it’s need to be :
api compatibility is set to : net 2.0 (not the subset)
and Stripping Level disabled

 using UnityEngine;
 using System.Collections;
 using System;
 using System.Net;
 using System.Net.Mail;
 using System.Net.Security;
 using System.Security.Cryptography.X509Certificates;
 
 public class mono_gmail : MonoBehaviour {
 
         void Main ()
         {
             MailMessage mail = new MailMessage();
 
             mail.From = new MailAddress("youraddress@gmail.com");
             mail.To.Add("youraddress@gmail.com");
             mail.Subject = "Test Mail";
             mail.Body = "This is for testing SMTP mail from GMAIL";
 
             SmtpClient smtpServer = new SmtpClient("smtp.gmail.com");
             smtpServer.Port = 587;
             smtpServer.Credentials = new System.Net.NetworkCredential("youraddress@gmail.com", "yourpassword") as ICredentialsByHost;
             smtpServer.EnableSsl = true;
             ServicePointManager.ServerCertificateValidationCallback = 
                 delegate(object s, X509Certificate certificate, X509Chain chain, SslPolicyErrors sslPolicyErrors) 
                     { return true; };
             smtpServer.Send(mail);
             Debug.Log("success");
         
         }
 }