Smtp exception icloud

Hello everyone,
I have a problem with smtpServer connection, as soon as it tries to send the message this happens: SmtpException: 550 5.1.1 Mailbox does not exist.
The code is as follows.

using System.Collections;
using System.Collections.Generic;
using System.Net;
using System.Net.Mail;
using System.Net.Security;
using System.Security.Cryptography.X509Certificates;
using UnityEngine;
using UnityEngine.UI;

public class Emailer : MonoBehaviour
    public InputField EmailReceiver;
    public int RandomCode;
    [SerializeField] private string CodeVerificationCheck;
    [SerializeField] private string EmailReceiverString;

    public void SendEmail()
        MailMessage message = new MailMessage();
        SmtpClient smtpServer = new SmtpClient("");
        smtpServer.Timeout = 10000;
        smtpServer.EnableSsl = true;
        smtpServer.DeliveryMethod = SmtpDeliveryMethod.Network;
        smtpServer.UseDefaultCredentials = false;
        smtpServer.Port = 587;

        smtpServer.Credentials = new System.Net.NetworkCredential(mail, password);
        ServicePointManager.ServerCertificateValidationCallback = delegate (object s, X509Certificate certificate, X509Chain chain, SslPolicyErrors sslPolicyErrors)
            return true;

        RandomCode = Random.Range(1000, 100000);
        message.From = new MailAddress("mail");
        message.To.Add(new MailAddress(EmailReceiver.text));
        message.Subject = "Code";
        message.Body = "Code " + RandomCode.ToString();

        message.DeliveryNotificationOptions = DeliveryNotificationOptions.OnFailure;

How can I solve it? Thank you.