Flagged as spam but accepted

[Adam Moffett]

2010-07-08 09:05:01 1OWqmi-0005N3-JU /*SA: Action: flagged as Spam but 
accepted: score=4.0 required=4.0 */(scanned in 0/0 secs | Message-Id: 
20100708130436.52c7d1cb1...@mail.microton.com.br). From 
<care...@habitat.com> (host=NULL [189.26.124.122]) for a...@plexicomm.net

The above is a line from my Exim log file.  Does anyone know under what 
conditions a message can be "flagged as spam but accepted"?

Thanks,
Adam