Help mailserver used to mirror spamming !!!

[Ricardo Fitzgerald <axis@movinet.com.uy>]

Hi to all,

Recently I had a very bad news one of my clients mail server was blocked due
to spamming !!! Well, the thing is they are not spamming at all, but their
ip was used by some unscrupulous spammer, now I have the task to write some
security to prevent that but I don't know much about the subject, where can
I find some info to develop a good firewall or sendmail rules to prevent
that from happen again ? I've recently started to use sendmail and it's
somewhat obscure, due to my personal situation I can't even afford to buy a
book on sendmail so I need free sources.
Any suggestions from experienced users are welcomed, now I have sendmail,
and fetchmail, procmail and squid as proxy to serve the internal lan of 5
windows computers.

TIA,

Rick