Email functionality not working in C#

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email

SMTP

I am trying to implement automatic email functionality for a website I developed. On initial basis I tested email code with Gmail credentials and it worked fine. Below is the code which is working fine.

public static int SendEmail(string from, string to, string subject, string body)
  {
      int result = 0;
      MailMessage msg = new MailMessage();
      MailMessage mail = new MailMessage();
      SmtpClient SmtpServer = new SmtpClient();
      SmtpServer.Credentials = new System.Net.NetworkCredential("MyEmailID@gmail.com", "MyGmailPassword");
      SmtpServer.Port = 587;
      SmtpServer.Host = "smtp.gmail.com";
      SmtpServer.EnableSsl = true;
      mail = new MailMessage();
      try
      {
          mail.From = new MailAddress(from, "", System.Text.Encoding.UTF8);
          mail.To.Add(to);
          mail.Subject = subject;
          mail.IsBodyHtml = true;
          mail.Body = body;
          mail.DeliveryNotificationOptions = DeliveryNotificationOptions.OnFailure;
          mail.ReplyToList.Add("MyEmailID@gmail.com");
          SmtpServer.Send(mail);
          result = 1; //--------- MESSAGE SENT SUCCESSFULLY
      }
      catch (Exception ex)
      {
          result = 2; //--------- ERROR OCCURRED WHILE SENDING MESSAGE
      }
      return result;
  }

When I used client's email settings with above code, it is not working. Client has provided me with below information.

Official EmailID/Password: Two different email address and their passwords
SMTP:
Outgoing Server: Two different server for SSL and NON SSL settings
SMTP PORT: 587 and 465

I tried all the possible combination of email/password, SMTP port..etc but none is working.

I am getting two kinds of error with these combinations...

err 1: System.Security.Authentication.AuthenticationException: The remote certificate is invalid according to the validation procedure.
err 2: The operation has timed out.

But I am able to login on webmail with the credentials provided, so I guess both the emailIDs, and their respective passwords are correct.

Any help would be appreciated.

Thanks and regards,

Sam