DomainKeys Identified Mail (DKIM) is an email authentication method designed to detect forged sender addresses in emails, which is a technique often used in phishing and email spam.
Setting up DKIM authentication is optional but highly recommended. It helps to ensure that emails aren't filtered as spam, especially for internal messages (where the sender email address and the recipient email address is from the same domain, for example from email@example.com to firstname.lastname@example.org).
DKIM authentication will be available as of April 1, 2020.
Related article: SPF records