Hello,
This is functionality that the IP Bypass list gives you. When an IP is on the spam IP Bypass list the next IP in-the-chain, using the received line, is checked instead of the immediately incoming IP. This allows for checks like SPF/DKIM/RBL's to check your true sender (the one sending to your gateway). However, I have not used version 16 in a while so my memory might be a bit foggy on if we've made fixes or changes to that area. If this solution does not work for you post back and we can figure something out.
Matt Petty
Senior Software Developer
SmarterTools Inc.
www.smartertools.com