Note to self: If you get error code 12250 from ISA server 2006…
“Error Code: 403 Forbidden. ISA Server is configured to block HTTP requests that require authentication. (12250)”
Everything works from and to the protected networks but traffic from the external network is blocked with the error response above.
Solution: Stop using the darn “Publish web site” wizard and copy already existing stuff 
-or- make sure that the “Allow client authentication over HTTP” check box is checked.
Found at the HTTP Listener properties – Authentication tab, click Advanced.
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