I fear I'll be switching off the gray list for the time being. I tried it today, and it sadly failed my simplest of tests.
Eight hours ago, I sent an email from a whitelisted address to a spamcop filtered email account and it still hasn't arrived.
I love the idea of the gray filter but until it has a few safeguards to ensure legit mail hasn't been lost or delayed beyond a reasonable time, I'll wait to implement.
JRS