When setting up your Flowmailer account, you'll have to add a domain to start sending emails from. To do so, the Domain Setup Wizard will guide you through the process. One of the most important steps is to add the generated CNAME records to your DNS. Optionally, you can also add the DMARC & SPF record to your DNS.
The essential DNS records will look like the image below.
The Name field is based on the domain and return-path domain you're setting up for the domain you want to authenticate. The Type is always CNAME. The dynamic Value fields are based on your account ID and domain name, letting Flowmailer generate the necessary authentication.