Email not being sent but no specific errors are showing

Here is the thing. I have been trying to send an email with the second method listed here: https://unity3dtuts.com/how-to-send-email-from-unity3d/

• I create a MailMessage and SmtpClient classes and populate them exactly how that wonderful post say, and it works! However, when I try the same thing with Siteground mail solution, it doesn't, and the only error I get from the callback is "Message could not be sent."

• I have input bad credentials in both Google and Siteground's smtp, and while Google tells me bad credentials where inputted, Siteground always respond with "Message could not be sent."

• From scavenging weird Siteground documentation I learned they use SPF and DKIM certifications, but I can't replace current X509 certification with these because (I guess) Unity doesn't include them by default.

• So, what should I try next?

     public static void SendMail(string to, string subject, string body)
       {
           Debug.Log("[Mail] Send mail to: " + to);
   
           MailMessage mail = new MailMessage();
           mail.From = new MailAddress(MailSenderUser);
           mail.To.Add(to);
           mail.Subject = subject;
           mail.Body = body;
   
           SmtpClient smtpClient = new SmtpClient(MailSenderSmtp);
           smtpClient.Port = MailSenderSmtpPort;
           smtpClient.Credentials = new NetworkCredential(MailSenderUser, MailSenderPassword) as ICredentialsByHost;
           smtpClient.EnableSsl = true;
   
           smtpClient.SendCompleted += new SendCompletedEventHandler(SendCompletedCallback);
   
           try
           {
               // ServicePointManager.ServerCertificateValidationCallback =
               // delegate (object s, X509Certificate certificate, X509Chain chain, SslPolicyErrors sslPolicyErrors)
               // { return true; };
   
               // ServicePointManager.ServerCertificateValidationCallback = delegate { return true; };
   
               smtpClient.SendAsync(mail, "Mail");
               Debug.Log("[Mail] Try send mail async");
           }
           catch (Exception exception)
           {
               Debug.LogWarning("[Mail] Exception sending email: " + exception.Message);
           }
       }
   
       public static void SendCompletedCallback(object sender, AsyncCompletedEventArgs args)
       {
           if (args.Error != null)
           {
               Debug.Log("[Mail] Error: " + args.Error.Message);
           }
           else
           {
               Debug.Log("[Mail] Mail Sent!");
           }
       }
   }