I really worry about saying to the spammer
"this site does greylisting" and, if that happens enough, having
the spammer respond (internally) "ah, it is greylisting and not
some random temporary server unavailability, we know what to do
giving spammers the information they need to become
smarter and the incentives to do so are really not in our
long-term best interests.
Greylisting only works due to spammer laziness. It is only a barrier for the
trivial spamming engines.
As soon as a spammer is motivated to do interesting work, they can and do work
around greylisting easily.
So this particular context is not one that warrants worrying about "giving"
information to spammers.
The reason that greylisting continues to be useful is that there happens to be a
significant portion of the spamming world that does, in fact, use trivial engines.