It makes no sense to greylist any host that properly retries to send the 
email. Each of those hosts might be whitelisted as well so that legitimate
emails can pass without delay.
Some services create lists of such hosts, for example:
	list.dnswl.org	(this service is only free for non-commercial users)

Each service needs to have its own line in file:
	/etc/greylistd/dnswl-known-good-sender
