What triggers an SPF Hard Fail?

An SPF Hard Fail occurs when an email is sent from an IP address that is not listed in the SPF record of the sending domain. SPF, or Sender Policy Framework, is an email authentication method used to prevent unauthorized senders from sending emails on behalf of a domain. When the receiving mail server checks the SPF record and determines that the IP address of the sender is not listed, it indicates a clear noncompliance with the sending domain's defined policy. This results in an SPF Hard Fail, which typically leads to the message being rejected or marked as spam.

The other options do not trigger an SPF Hard Fail in the context of email authentication. The status of the recipient's server being down, the presence of a bad attachment, or the lack of DKIM (DomainKeys Identified Mail) signatures are issues that may affect email delivery but are not related to the SPF authentication process. Therefore, the essential point about SPF Hard Fail is directly tied to the sender not being from an authorized IP address as outlined in the domain's SPF record.