Yahoo uses DomainKeys Identified Mail to detect forged emails
We want you to feel safe when opening emails. As part of our efforts, Yahoo Mail fights email forgery with DomainKeys. What is DomainKeys? We're glad you asked.
DomainKeys Identified Mail (DKIM) lets senders digitally sign their emails sent to Yahoo Mail accounts. This enables Yahoo Mail to verify the sender's authenticity and encrypts the message so its origin (domain) can be checked.