Before sending out any letters to your customers, you have to verify your From address' domain. You can't launch campaigns, enable automations or use API endpoints, from a domain in your From addresses, which isn't verified.
You can verify you domain by going under your account preferences.
Open "Domains" (1) tab and then click on "verify domain" (2).
A pop-up window will appear that asks you to enter the email address. Enter an email address that contains the domain you wish to verify and then click "Send verification email".
After clicking the button, you will receive a verification email on the same email address you entered. In the email you can either click on the "verify domain" button or copy-paste the given link to your browser address bar.
If everything is ok, you should now see under the domain tab in you account, that your domain has been verified and you can start sending out letters.
Also you can see and manage all the domains that have been verified or pending verification.
If you have any questions regarding domain verification, please contact firstname.lastname@example.org.