DKIM (DomainKeys Identified Mail) - an email authentication method based on verifying the authenticity of a digital signature. The public key is stored in the TXT record of the domain.
DKIM is required so that postal services can check whether the sender is valid or not. That is, it protects the recipient of the letter from various fraudulent letters sent with the substitution of the sender's address.
To enable DKIM, do the following:
"v=DKIM1; k=rsa; s=email;to the end, including the character
"two lines below).