Error "The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.5.1 Authentication Required"

"The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.5.1 Authentication Required"

I know the username and password are correct and I can login via the web interface an send email via this account no problems. Here is my code - can any body shed any light on why this would happen? Putting an invalid password gives the same error. I also have disabled TFA.

	SmtpClient smtp = new SmtpClient("smtp.zoho.com", 587);
	smtp.EnableSsl = true;
	smtp.UseDefaultCredentials = false;
	smtp.Credentials = new NetworkCredential("user@domain.com", "password");

	MailMessage msg = new MailMessage("user@domain.com", "toaddress@adomain.com");
	msg.Subject = "hello";
	msg.Body = "world";

	smtp.Send(msg);

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