Check your DKIM
Check DKIM record of your domain!
What is DKIM?
DomainKeys Identified Mail (DKIM) is an anti-tamper protocol that ensures your mail remains secure in transit. DKIM uses digital signatures to check that the email was sent by a specific domain. Furthermore, it checks if the domain authorised the sending of the email. In that, it is an extension of SPF.
DKIM uses two actions to verify your messages. The first action takes place on a server sending DKIM signed emails, while the second happens on a recipient server checking DKIM signatures on incoming messages. The entire process is made possible by a private/public key pair. Your private key is kept secret and safe, either on your own server or with your ESP and the public key is added to the DNS records for your domain to broadcast it to the world to help verify your messages.
Image Credit: Global Cyber Alliance