Sending › DNS check has two tools:
Your Domains: shows the DNS status of every sending domain you connected, whether it can send, and when it was last checked. Check Now re-checks one domain, Refresh All checks them all.
External Domain Checker: checks any domain, even one not connected to your account, using public DNS. Type the domain and press Check; you get a health score, current and expected values, and recommendations.
If you turn on Email Alerts on the same page, you get an email when a DNS problem is detected on your domains.