Some posters are being deleted with this error message: The  last  reported  
error  was:  5.7.1  554  5.7.1  optonline.net  is  blacklisted. 

I turned off the automatic delete feature for the list, but I'm confused about 
this error. So someone with a non optonline address posts to the list which 
contains 30 or so subscribers with optonline addresses. The poster is being 
punished because those addresses are blacklisted? I don't have any blacklists 
configured on the server, so I'm not sure where its getting its information. 
Incoming does go through a barracuda appliance.  Is something more sinister 
happening here?