Milter-greylist

All -

There's a happy flame-fest (greylist rules vs. greylist sucks) going on
on my LUG mailing list, and a question has arisen which I can't answer.

We can set the retry interval in /etc/mail/greylist.conf file (i.e.
"greylist 1h"), and the error message which is sent back to the sending
server looks something like "reject=451 4.7.1 Greylisting in action,
please come back in 00:01:00"

Is the text after the 451 error code actually interpreted by the sending
MTA?  Or is it just human readable info for the sysadmin reading log
files on the sending side?

The reason I ask is, I've seen Exchange servers with the default 15
minute retry configured re-send within just a few minutes (i.e. less
than 5) when I set "greylist 1m" in my greylist.conf.

Am I smoking crack?  What is the scoop here?

Thomas