Mailbox unavailable ... not permitted to relay

I am debugging sending email using information from a web page form.
The method I am using is very similar to the KB article:
http://support.Winhost.com/KB/a650/how-to-send-email-in-aspnet.aspx?KBSearchID=365406

When I send I get the error message Exception Details: System.Net.Mail.SmtpFailedRecipientException: Mailbox unavailable. The server response was: w07.Winhost.com (W07) [96.31.35.71] is currently not permitted to relay

I believe this is a problem with the authentication of the SMTP account. I do set credentials before I send the mail as in the KB article. I use an email account = [email protected]. The relevant code snippet is below-

Dim client As New SmtpClient()
Dim basicAuthenticationInfo As New System.Net.NetworkCredential("[email protected]", "thePasswordHere")
client.Host = "mail.sailfastllc.com"
client.UseDefaultCredentials = False
client.Credentials = basicAuthenticationInfo
client.Send(message)

I know the email account is working because I can use the site email manager and successfully send an email to the same address I'm trying to send to in code.

I am in process of transferring the site from another host to Winhost. So I am doing all this using the secondary URL.

Any ideas ??

Kevin