Paul Russell <[log in to unmask]> wrote on 02/22/2007 10:16:53 PM:

> Generally, server-side filters filter on the envelope-sender address, 
not the
> 'From:' address. The 'From:' address is not available for server-
> side processing
> until the mail server accepts the entire message, because the 
> 'From:' address is
> part of the DATA portion of the SMTP transaction.

I'm want to clarify something in the above.  The From: (in the header) is 
not available iuntil the server *RECEIVES* the entire message.  It may or 
not accept the message after receiving it.  There are a number of spam 
filters that process the message before returning an accept or reject to 
the DATA phase of the connection.  Google for sendmail and milter for 
information and examples.  
http://www.google.com/search?q=sendmail%20milter