Check if you can ping mail.btconnect.com. If not run a tracert on it to see where it fails. There might be no route to that SMTP server.
mail.btconnect.com uses port 25, ALD2355
Not had any Other Experiance with Interspire sadly,
Just another one try using SMTP.btconnect.com (with the same settings "Port" 25) and see if that helps