Milter-greylist

I have this strange problem and was wondering if anybody has 
experienced anything like it.  A sender's Outlook server seems to not to 
be able to handle greylisting.   Their messages were getting bounced, so 
I first whitelisted the sender, then took the normal recipient off 
greylisting.

Things usually work, but sometimes the sender's server, not my server, 
kicks the messages back.  It seems to be some sort of residual effect of 
greylisting where something's stuck in a cache or a queue somewhere.  
The only person who has any problems receiving email from this sender is 
someone who was once on greylisting.  Weird.