DomainKeys Identified Mail (DKIM) is a method of digitally signing emails so that the recipient mailbox provider can run a test and see if the message headers or body were altered in-transit. It is used for fraud pevention.
There are two components to DKIM, a special DNS record containing the public key and a corresponding secret key stored within the sending software or by the Email Service Provider (ESP).
Having properly configured DKIM will improve your deliverability, especially if you are bulk sender.
Not every ESP supports DKIM so it’s best to check in advance before you place your order. Make sure to check that they support DKIM for all recipient addresses, not just those belonging to large providers.