sonic surrounds

…..For security reasons Postfix allows users defined in mynetworks to send emails. Usually your mail server will only accept mails for its own domains. If you allowed everybody to send email to every other domain you would provide a so called open relay that spammers abuse to send out their digital trash……
http://workaround.org/articles/ispmail-sarge/#id2491961

http://www.secinf.net/unix_security/Securing_Debian_HOWTO/

Have a look back at this site for HOWTOs on securing a new debian install. Note however that these files seem to be pre-Woody, so could be worthwhile looking around for newer info as well.