What triggers an SPF Hard Fail?

An SPF Hard Fail is triggered specifically when an email is sent from an IP address that is not listed among the authorized senders for the domain. Sender Policy Framework (SPF) is designed to help fight email spoofing by allowing the domain owners to specify which mail servers are permitted to send emails on their behalf.

When an email is sent from an unauthorized IP address, the receiving mail server checks the SPF record of the sender's domain. If the IP address is not included in this record, the result is a hard fail. This means the email is outright rejected because it is seen as potentially fraudulent or spoofed.

This mechanism is critical for improving email security, as it helps safeguard against phishing attacks and protects the integrity of email communications by ensuring that only authorized senders can deliver mail for a given domain.