There are several key protocols used to authenticate emails:
Sender Policy Framework (SPF): SPF allows the owner of a domain to specify which mail servers are permitted to send emails on its behalf. This helps in preventing unauthorized use of the domain. DomainKeys Identified Mail (DKIM): DKIM provides a way to validate a domain name identity that is associated with a message through cryptographic authentication. It adds a signature header to the email that can be verified by the recipient's email server. Domain-based Message Authentication, Reporting & Conformance (DMARC): DMARC builds on SPF and DKIM to provide a way to validate that an email comes from the domain it claims to be from. It also gives domain owners a reporting mechanism to understand how their emails are being handled by ISPs.