System.net.mail.smtpexception: 'the SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.7.0 authentication required. Learn more at'

<pre>public Task SendAsync(IdentityMessage message)
        {
            // Plug in your email service here to send an email.
            var mailMessage = new MailMessage();
            mailMessage.To.Add(new MailAddress(message.Destination));
            mailMessage.From = new MailAddress("gcobanim@mandela.ac.za");
            mailMessage.Subject = message.Subject;
            mailMessage.IsBodyHtml = true;
            mailMessage.Body = message.Body;
            using (var smtp = new SmtpClient())
            {
                var credential = new NetworkCredential
                {
                    UserName = "ggcobani@gmail.com",
                    Password = "Telkom35@@"
                };
                try
                {
                    smtp.Host = "smtp.gmail.com";
                    smtp.Port = 587;
                    smtp.EnableSsl = true;
                    smtp.UseDefaultCredentials = false;
                    smtp.Credentials = credential;
                    smtp.DeliveryMethod = SmtpDeliveryMethod.Network;
                    smtp.Send(mailMessage);
                }
                catch (Exception ex)
                {
                    throw;
                }
            }
            return Task.FromResult(0);
        }
    }