DKIM

DKIM (Domain Keys Identified Mail) is an email authentication process that grants the receiver of an email the ability to confirm that it was authorized and sent by the true owner of the sending domain. This is accomplished by embedding an encrypted digital signature in the header of the Email.

After which, the receiving mail server confirms that the email is signed with a valid DKIM signature, and that the constituent parts of the email, including the message body and attachments, have not been altered. DKIM signatures are not visible to the end-users since they are typically validated at the server level.