Thanks for the reply, Jeff.
My problem, tho, is that senders seem to be bypassing the greylisting. They send to my primary MX IP, and when they receive the greylist-fail response, they immediately send to my secondary MX IP (the backup mail service), ignoring the "send again in x seconds" delay. I'm assuming since the secondary MX IP is in the Bypass Gateways list, Smartermail is just letting the message go straight thru.
I'm not looking to bypass greylisting for the backup mail service. Rather, if a message is greylisted when sent to the primary MX IP, Smartermail should recognize the same message coming into the secondary MX IP and reply with another "send again in x seconds" response. Maybe I should try to remove the secondary IPs from Bypass Gateways and see how the messages fail. I had it set like that originally, but can't recall exactly why messages weren't coming thru the backup server. much confuse.