It's just a guess, but I'm thinking that the Envelope-From may be using your domain name which is why it is getting whitelisted. Is it possible to get your MTA to include the Envelope From in the headers?
Do you know how I can do that? I am running postfix. I could check the logs for that date and see what was put there.
If you know of a setting to set to include envelope header values in the header somewhere, I'd turn that on.
