stopping my server from spamming

[David Banning]

> This is more of a question geared towards your mail server application than FreeBSD. You should check your mail logs. If you want better advise, you may want to provide more information on what mail server are you running, and what did you do to prevent SMTP relay.

I am using sendmail. It will not allow open relaying. What I would
like to know is
how I can separate legitimate emails in the log from spam. All that
appears is the from:
<email> and the to:<email>.

In the past I have seen separate SMTP servers installed by viruses on
windows boxes
which are spamming away -independent- of sendmail. I have blocked port
25 from all
my connected windows boxes, but will that take care of it?