Spam is a nuisance - this is a fact that I won't deny - so I can't blame people for using spam filtering software to keep their inboxes clean. One measure that a system administrator can employ is the filtering of incoming mail against DNS blocklists - the sending servers are looked up against another machine which reports back whether the server is a known spam sender. At this point, the receiving machine can use that information to either reject the mail outright, or use the information as part of a scoring mechanism.
At work, the system administrator inbox seemed to have a load more "Delivery Failed" meaages generated than normal so I decided to take a look to see what the cause could be. After much digging, I found that our corporate domain had been listed on the SURBL WS list.