A delivering email system is configured to receive a request to send an email to a recipient, identify an authentication method of a sender account for the email, modify email headers of the email to include an indication of the authentication method, generate digital signatures for the email that include the email headers within a scope of the digital signatures, modify the email such that an email header of the email includes the digital signatures, and transmit the email, including the indication of the authentication method and the digital signatures, to the recipient at a receiving email system. The receiving email system is configured to receive the email, determine that the email headers are unaltered by validating the digital signatures against a public key of the sender domain, determine whether the authentication method indicated meets a criteria, and execute a security response against the email if not.