I have found that recently a spammer has been using a mod_proxy 
configuration, (that was meant to allow for an easier transition to a 
new naming scheme, as well as changes to a backend software) as a spam 
relay.  
The spammer has been using HTTP POST requests to send these messages
with POST HTTP://mailserver:25/ HTTP/1.1  
With some research it looks like this is an automated process including 
the initial scan stage.

When I contacted Apache in regards to this, the response was not very 
promising. 

This problem would be a simple fix with implementing the AllowConnect 
configuration option within proxy_http, to prevent outbound 
connections.  

Jason