Mailgun Successfully Configures DNS but Is Disabled

Mailgun is configured for authentication mail for Mastodon.

The DNS server is FreeDNS, set the records as shown in Mailgun's Domain Verification & DNS, and press the Check button to check each record properly.

However, the top state remains disabled.

I've also read the FAQ site of Mailgun, but I'm having a hard time understanding it.

What should I do to make it work?