stopping my server from spamming

[David Banning]

I have been blacklisted for spamming and I am attempting to ascertain the
source.

I have a few networked windows boxes which route through a FBSD
server. I also have around ten off-site users who send mail via my port
26 - using smtp password authentication. How do I determine which
email going through the server is spam, and therefore identify the
source?