CS0433, No PUN. Unity 5.4. C#.

Title basically gives all details.

Error Codes: Assets/Scripts/GmailScript.cs(59,17): error CS0433: The imported type `System.Net.Mail.SmtpClient' is defined multiple times

Script:

 using UnityEngine;
 using UnityEngine.UI;
 using System.Collections;
 using System;
 using System.Net;
 using System.Net.Mail;
 using System.Net.Security;
 using System.Security.Cryptography.X509Certificates;
 
 public class GmailScript : MonoBehaviour {
 
     public Text User_nameT;
     public string User_name;
 
     public Text User_MessageT;
     public string User_Message;
 
     public float coolDown = 60;
     public float Timer;
 
     public Text SubmitText;
 
     void Update()
     {
         if (Timer > 0)
         {
             Timer -= Time.deltaTime;
             SubmitText.text = Mathf.Round(Timer).ToString();
         }
 
         if (Timer < 0)
         {
             Timer = 0;
             SubmitText.text = "SUBMIT";
         }
 
         if (Timer == 0)
         {
             SubmitText.text = "SUBMIT";
         }
 
     }
 
     public void Send()
     {
         User_Message = User_MessageT.text;
         User_name = User_nameT.text;
         if (MenuUIMan.TogVal == 0)
         {
             if (Timer == 0)
             {
                 MailMessage mail = new MailMessage();
 
                 mail.From = new MailAddress("Receiver@gmail.com");
                 mail.To.Add("Sender@gmail.com");
                 mail.Subject = "BugReport_GivenFromComBot";
                 mail.Body = "User:" + User_name + "          " + "Message:" + User_Message;
 
                 SmtpClient smtpServer = new SmtpClient("smtp.gmail.com");
                 smtpServer.Port = 587;
                 smtpServer.Credentials = new System.Net.NetworkCredential("Sender@gmail.com", "SenderPassword!") as ICredentialsByHost;
                 smtpServer.EnableSsl = true;
                 ServicePointManager.ServerCertificateValidationCallback =
                     delegate (object s, X509Certificate certificate, X509Chain chain, SslPolicyErrors sslPolicyErrors)
                     { return true; };
                 smtpServer.Send(mail);
                 Debug.Log("success");
 
                 Timer = coolDown;
             }
         }
 
         if (MenuUIMan.TogVal == 1)
         {
             if (Timer == 0)
             {
                 MailMessage mail = new MailMessage();
 
                 mail.From = new MailAddress("Receiver@gmail.com");
                 mail.To.Add("Sender@gmail.com");
                 mail.Subject = "Suggestion_GivenFromComBot";
                 mail.Body = "User:" + User_name + "          " + "Message:" + User_Message;
 
                 SmtpClient smtpServer = new SmtpClient("smtp.gmail.com");
                 smtpServer.Port = 587;
                 smtpServer.Credentials = new System.Net.NetworkCredential("Sender@gmail.com", "SenderPassword!") as ICredentialsByHost;
                 smtpServer.EnableSsl = true;
                 ServicePointManager.ServerCertificateValidationCallback =
                     delegate (object s, X509Certificate certificate, X509Chain chain, SslPolicyErrors sslPolicyErrors)
                     { return true; };
                 smtpServer.Send(mail);
                 Debug.Log("success");
 
                 Timer = coolDown;
             }
         }
     }
 }