Hello,
One of the first places you can look is the IP Bypass under settings -> antispam. If the IP is listed here it will be skipped for your spam checks and the next IP in line ( the private IP in this case ) will be scanned, If there are no more IPs in teh header the scan will end.
I hope this helps. Thank you
Tony Scholz
System/Network Administrator
SmarterTools Inc.
www.smartertools.com