DomainKeys Identified Mail (DKIM) is used to authenticate the emails.
if they are not authenticated ones they are redirected to spam.
DKIM help any company who sends email to be responsible for the email they send.
What does DKIM uses for authentication ?
- Domain name identifier , which is unique to any domain (domains are individual websites)
- Public key cryptography
- DNS based public key publishing service
A mail emerging from any domain is signed by Mail user agent or Mail submission agent along the way to sender
Once the email is sent , email provider service (like gmail ) will verify with DNS.
The ultimate goal of this framework is to permit a signing domain to assert responsibility for a message, thus protecting message signer identity and the integrity of the messages.
RFC text : https://www.ietf.org/rfc/rfc4871.txt
Go through FAQ regarding DKIM :
Example DKIM sign :
DKIM-Signature a=rsa-sha1; q=dns;
where b is signature which is unique to the DNS of sender