DomainKeys Identified Mail (DKIM) is a protocol for signing outgoing email messages with a signature that proves it really comes from the sender's domain. This can be used by recipient mail servers or spam filters to detect sender forgery, which is typically done by spammers to avoid having their domain blacklisted.

When DKIM is enabled using Virtualmin your mail server will be configured to add a DKIM signature to all outgoing messages, and a public key will be added to each of the DNS domains on your system. Other mail servers can then use this key to verify the signature on relayed messages, thus ensuring that the sender's address was not forged.

If you have virtual servers for which DNS is hosted elsewhere but mail is hosted by this system, you can enter their domain names into the Additional domains to sign for box. The records in the DNS records for additional domains box can then be manually added to those zones.