DKIM helps you assign a cryptic signature to your email header which in turn allows you to ensure that the email reaching your client is unaltered and genuine. In order to enable it for your domains on Drip, you need to set up a custom domain.
Set Up Your Custom Sending Domain
Drip allows you to set up a custom sending domain and publish CNAME records to enable SPF and DKIM for your domain. Click here to learn how.