What allows ISPs to handle emails that fail authentication checks?

The correct choice is Domain-based Message Authentication, Reporting and Conformance (DMARC). DMARC is a protocol that builds upon SPF and DKIM to provide a way for email senders and receivers to improve and ensure the authenticity of email messages. It allows ISPs (Internet Service Providers) to handle emails that fail authentication checks effectively.

DMARC works by enabling domain owners to publish policies that specify their email authentication practices and how their messages should be treated if they fail those checks. For instance, it can instruct ISPs to either quarantine or reject emails that do not pass SPF or DKIM validations. Additionally, DMARC offers reporting capabilities that help domain owners receive feedback on the mail that is being sent using their domain. This mechanism not only enhances the security of email communications but also facilitates better tracking and management of email authentication issues.

In contrast, while SPF and DKIM are essential components of email authentication, they operate independently and do not provide the same level of guidance for handling failed authentications as DMARC does. BIMI, on the other hand, is primarily focused on branding and visual identification rather than on the authentication handling process. Thus, DMARC stands out as the solution that specifically allows ISPs to manage emails failing authentication checks effectively.