Getting "Relay access denied" error - critical failure

Sending email via .Net SMTP class (in System.Net.Mail).
Send() method returns error:
Transaction failed. The server response was: 5.7.1 <chris.cardinal66@gmail.com>: Relay access denied

Have tried a couple of different email addresses. Same result.
C# Code is below:

MailMessage message = new MailMessage("XXXXXXXXXXXXXXXXXXXXXXX", "chris.cardinal66@gmail.com", "CloudServer check failure. ***URGENT***", msg);
SmtpClient emailClient = new SmtpClient("smtp.sendgrid.net");
System.Net.NetworkCredential SMTPUserInfo = new System.Net.NetworkCredential("YYYYYYYYYYYYYYYYYYYYY", "XXXXXXXXXXXXXXX");
emailClient.UseDefaultCredentials = false;
emailClient.Credentials = SMTPUserInfo;
emailClient.Send(message);

Looks like your server is not relaying.