There is a method to bypass RBL checking for certain addresses and domains in Postfix. This assumes that you are using RBL checking in the first place.
First, create an rbl_override file in /etc/postfix.
Place the domains and/or IP addresses you would like whitelisted into that file.
Create the hash of that file.
Edit the main.cf file.
Locate the section marked smtp_recipient_restrictions. You need to add the following to that.
Restart Postfix and you should be all set.