When I send email to some of my contacts, I have been getting a bounce message indicating my email was being treated as spam. This is due to my IP addresses being in a range of IPs on a Realtime Blackhole List (RBL). The idea of an RBL is simple - if you send spam, your IP gets tagged as a spammer and added to a list. Other responsible mail admins can "subscribe" to the list and then block any messages from your IP. This is just one of many methods to fight spam, but can sometimes be controversial - as I found out.