What is DKIM?
DKIM (DomainKeys Identified Mail) is a method that associates domain names with email addresses. This allows validation of the authenticity of email senders.
DKIM is based on asymmetric encryption. The sending email server generates a message header with a cryptographic signature of the message content, which the receiving server verifies using a public key registered in the domain’s DNS. If the calculated hash and the one contained in the email signature match, it can be assumed that the message comes from the genuine sender. Furthermore, it can be assumed that the message text has been transmitted unchanged to the recipient.
hosting.fr mail servers sign all messages under the “mailerdienst.de” domain by default.
Complete documentation on DKIM is available at opendkim.org.