One way SpamFilter could do this would be to create a thread periodically to add the whitelisted IP's (not already on the greylist) to the greylist. This can be done for all domains in the enterprise version or just the one whitelist for standalone.   One challenge would be to deal with globals properly since the greylist likely only uses individual IP addresses.  The other challenge would be to handle file locking and multi thread file access accurately.  This would not be a thread that would need to run very often, maybe even a little as once per hour.