How to solve " STARTTLS command first"?

I am getting exceptions durring sending email.

My code

var message = new MimeMessage();

message.From.Add(new MailboxAddress(from));

message.To.Add(new MailboxAddress(to));

message.Subject = "Simple mail'?";

message.Body = new TextPart("plain")

{

Text = body

};

using (var client = new MailKit.Net.Smtp.SmtpClient())

{

// For demo-purposes, accept all SSL certificates (in case the server supports STARTTLS)

client.ServerCertificateValidationCallback = (s, c, h, e) => true;

client.Connect("smtp.gmail.com", 587, SecureSocketOptions.StartTlsWhenAvailable);

// Note: since we don't have an OAuth2 token, disable

// the XOAUTH2 authentication mechanism.

client.AuthenticationMechanisms.Remove("XOAUTH2");

// Note: only needed if the SMTP server requires authentication

client.Authenticate("[email protected]", "password");

try

{

client.Send(message);

client.Disconnect(true);

}catch(Exception ex)

{

}

Exceptions: SMTP server does not support secure Authentication.

if comment client.Authenticate("[email protected]", "password"); then getting this exception,

"5.7.0 Must issue a STARTTLS command first. v64sm22788514pfk.86 - gsmtp"

please let me know how is coming and where am i wrong.

Thanks and Reagards

Sameer Shaikh