Milter-greylist

Hello everybody,
I just installed milter-greylist (easy, thanks for the good doc).

But, I have a problem (I think):

The MX record for my server, say myserver.org is handled by the MTA of
my ISP, say myisp.org.

The ISP MTA cleans-up viruses and some spam but NOT all, so I decided
to implement Greylisting. My ISP is not planning to implement it.

So, the problem is that I need to tell milter-greylist that I receive
mail mostly from that 'peer' and therefore send 'delayed messages' to
the real IP of the sender.

Setting up 'peer 123.123.123.123' seems to work (I now see the sender
real IP in the greylist accessdb), but obviously the sync won't work
'cause myisp.org has no milter-greylist.

I do not understand if in such a way I'm "talking" directly to the MTA
of the sender (that will "answer" then directly to me) or what else is
happening.

Is there a workaround for that?

May I just ignore no-sync messages? (By the way, my ISP won't be that
happy if I just try to do useless things with their server....)



Thank you and regards,
nIc