Milter-greylist

I have lots of "autowhitelisted entry expired" error nowadays. This error shows up over a hundred times a minute on each server.
When this happens, the message does not deliver to incoming emails.
I have one legit email with "autowhitelisted entry expired" error since 8am today and now it's 5:40pm already still not showing up in my inbox. 
Chances are that the message will never be delivered.

Does anyone know how to minimize or fix this problem?

Thanks,
An Nguyen