I have several sites, one of which is on zinc, one on iridium. Using php's mail(), an email was sent by my Content Management System (XOOPS) from the domain on iridium to the domain on zinc. The sending domain received the email back with the message:

This message was created automatically by mail delivery software.

A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:

[email protected]

SMTP error from remote mailer after end of data:
host mydomain.com [Destination_IPADDRESS]: 550 Administrative prohibition
I checked the filters I have set up and none of them should have blocked it. I sent to other non-jag domains and they received them perfectly.

Any ideas? Thanks in advance...