IP Address: 63.251.72.156

The address is registered to PF IPV4 Borrower, LLC (UG-210), an enterprise network operator. Concentrated authentication failures on enterprise address space can indicate either a compromised internal host being used as an unauthorized sending relay, or an organization knowingly or unknowingly operating as a spam source.

Across the wider UG-210 network, 5 distinct IPs have been associated with 13 authentication failures over 13 observed messages, spanning 1 country. Most observed IPs on this network contribute to the failure count, suggesting the range as a whole warrants elevated scrutiny.

If your domain appears in the From header of mail from this address, treat it as probable spoofing. Verify that your SPF record does not authorize this host, directly or through nested include mechanisms, and that no DKIM selector you publish has been issued to it. If both checks come back clean, the receiver's reject action is doing its job.

Your DMARC policy posture matters more than any IP-level response here. The enforcement action applied to this mail indicates your policy is already providing protection. Maintaining p=reject across all your domains closes the gap for attackers who manage partial alignment. Domains that remain at p=none long-term tend to be impersonated repeatedly, because the cost to the attacker of attempting is effectively zero.

Blocking this individual address has limited durability: an attacker can rotate to another address in the same /24 subnet at effectively zero cost. More durable responses include monitoring aggregate DMARC reports so new sources are visible as they emerge, tightening SPF to remove overly permissive include chains or +all mechanisms, and ensuring DKIM is signing every legitimate outbound stream so alignment failures are unambiguous. The formal abuse contact for PF IPV4 Borrower, LLC is listed in ARIN WHOIS records, and timely remediation is achievable through that channel.