c# - SMTP error: "Client does not have permission to submit mail to this server"

I'm getting the following error while sending email. What could be the cause?

Client does not have permission to
submit mail to this server. The server
response was: 5.5.1 STARTTLS may not
be repeated.

Here's the stack trace...

Stack Trace

at System.Net.Mail.StartTlsCommand.CheckResponse(SmtpStatusCode statusCode, String response)
at System.Net.Mail.StartTlsCommand.Send(SmtpConnection conn)
at System.Net.Mail.SmtpConnection.GetConnection(String host, Int32 port)
at System.Net.Mail.SmtpTransport.GetConnection(String host, Int32 port)
at System.Net.Mail.SmtpClient.GetConnection()
at System.Net.Mail.SmtpClient.Send(MailMessage message)

I'm connecting to smtp.gmail.com with SSL on port 587 / 465