If you host your server on a dynamic IP address or DSL line, it is blacklisted almost automatically, no matter if you send spam or not.
If you have a static IP address, you should check your mail log if there''s much more email traffic than you expected, and you should also check your mailqueue with
to see if there are lots of emails in it (which might be a sign that your server is abused by spammers).
Also, what's the output of
postconf -d | grep mynetworks
postconf -n | grep mynetworks