Cisco
Cisco Secure Email is a spam filter, block list, and firewall solution for businesses. Mail is routed through mailservers owned by Cisco (iphmx.com) before it's passed along to the recipient domain's mailservers for final delivery. The recipient is able to manage a lot of delivery settings such a allow lists,so it helps to ask the recipient to run the described debugging tests in the user guide to identify and resolve delivery issues, as there is no way to contact the vendor directly.
Codes
- 550↑
550 5.7.1 Your access to submit messages to this e-mail system has been rejected.
This occurs when there is a block on something on the message; either your IP or domain have low reputation as reported by Cisco's Threat Intelligence or a manually set policy. You may want to contact the recipent through alternate ways so they can check the logs and add you to an allow list or contact Talos for a reclassification.