Sender-based exceptions to RBLs?

I have a vendor with whom I communicate that is currently blocked by my =
RBLs. I have every reason to believe the RBL block is legitmate based on =
other customers that use the same mail relay as he does.

What I'd like to be able to do is permit certain sender addresses (or =
domains) before hitting the RBL check.

I thought I'd be able to do a "check_sender_access maptype:map" check, but =
reading documentation it appears that's not a valid argument to smtpd_clien=
t_restrictions. I suppose it's important in which order the various =
checks are applied, so I guess my primary questions are:

What's the right way to exclude certain domains/e-mail addresses (not IP =
addresses) from RBL checks?

If use a smtpd_sender_restrictions access map with PERMIT for these =
domains, will that be checked before the RBLs or at least supercede the =
RBL?

Thanks for any help!

- Dave Alexander