#4 SMTP Send fails with good credentials

[Scott Stewart]

System.Net.Mail.SmtpException: Service not available, closing transmission channel. The server response was: Sorry, you must log in before using this server.
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)

occurs with credentials I have verified to work in Outlook.

Is there a way to debug the SMTP connect?