Understanding a Postfix log file entry

I'm not quite sure what you are trying to do here. I'd suggest reading the CONTENT_FILTER_README and SMTPD_POLICY_README in the Postfix documentation. Your test.php script should probably be running as a content filter rather than a delivery-time script. Especially if you are using the script to reject messages based on sender/recipient, it's considered bad form to make those sorts of decisions after you've accepted the message for delivery.

The usual way to do content filtering in Postfix is for the front-end smtpd to hand off to a content filter via SMTP, LMTP or a pipe, and the content filter to reinject into an entry point that has no content filtering enabled -- either another smtpd with the content_filter parameter disabled in a -o override in master.cf, or local submission using /usr/sbin/sendmail with the Postfix pickup daemon's content_filter disabled in master.cf.

This happens when domain.com has an MX record (or, in the absence of an MX, an A record) that points to your Postfix server, but your Postfix server is not configured to accept mail for that domain. It's commonly seen in two situations:

1. You've acquired a new domain that you want to use for email, but you just forgot to add it to Postfix. There are a couple ways to do it. In my case, I'm using the virtual table to map addresses to real local users, so I list my domains in the virtual_alias_domains directive in main.cf.
2. The domain has its MX record set to a host that resolves to 127.0.0.1. Some malware will use this trick so they can set their return address to a valid domain, which most anti-spam measures will check for. However, the MX effectively prevents any delivery to that address, so the sender doesn't have to deal with replies or bounces.