Sender Based Reputation Score

We recently implemented an Ironport eMail and web filtering solution when we switched ISPs.  It seems to be working very well but just in the last few days our Sender Based Reputation Score has shown  up as being poor.  So some sent emails have been rejected to vendors that I'm pretty sure are using the same IronPort solution which uses SBRS filtering.  So my question is - how do I find out why our score has turned to "poor".  I've checked our virus application and I'm not seeing any issues with viruses (at least that it reports).  Is there another way to check to see if a computer on our network may be sending out spam email?   There is no spike or anything alarming on our Ironport machines.  
Could it be something else causing this?  Any help would be very much appreciated.  I want to stop this before it gets worse - especially if its something on our network causing this.   Also - we are using a Cisco ASA firewall.

Thanks!