After creating your Transactional API key, the next step is to set up your sending domain. This tells Mailchimp which domain your transactional emails will be sent from, and it’s required before you can configure authentication with DKIM and DMARC.
Step 1: Go to Domain Settings
Log in to your Mailchimp Transactional dashboard.
From the menu, click Settings.
Select Domains.
Step 2: Add Your Domain
In the Add a Domain field, type the domain you want to use for sending (for example,
mystore.com).Click Add.
Step 3: Add DKIM and DMARC
Once your domain is added, you’ll need to set up DKIM and DMARC records to complete the authentication process. This ensures your emails are trusted by inbox providers and reach your customers’ inboxes instead of spam.
How to Configure DKIM for Your Sending Domain - Mailchimp ESP