Ignoring first hop (backup-mx)

[am.lists]

I'm on the verge of moving hosting providers.

Unfortunately, I'm moving the place where my two primary proxies live
(MailScanner 4.58 at the moment, upgrade comes soon, Postfix 2.2.2,
and ClamAV).

I have an agreement with a buddy of mine to provide me with backup-mx
services for the next couple of days while I do the move.

But... I don't know what his filtering has to offer (if any), and I
don't want to inadvertently take in a bunch of junk because I have to
w/l his box.

Once the move is finished and I'm up and running on the other side,
how do I whitelist his IP to receive all of that mail in for my users
without killing SPF rules and also being able to check that "2nd" hop
instead of the first hop (which will be his server) against my SA
rule-based RBLs?

Sorry if this is an obvious one, but it's not one I've had to deal with yet.

Angelo