Everything essential.
THE TWO PLACES
Nameservers: set at the registrar.
Records: held wherever those nameservers point.
Check which before editing anything.
THE RECORDS THAT MATTER
A, for the website MX, for mail, pointing at a hostname that itself resolves TXT, for SPF, DKIM, DMARC and verification
BEFORE CHANGING ANYTHING
Export the zone Lower TTLs a day ahead Know what must not change
AFTER CHANGING ANYTHING
Query the authoritative nameservers to confirm Test the site Test mail, both directions Check subdomains
WHEN MAIL STOPS
MX records missing, the mail hostname not resolving, or cPanel routing set wrong.
WHEN NOTHING RESOLVES
Domain expired, nameservers wrong, or no A record.
WHEN A CHANGE SEEMS NOT TO WORK
Usually made in the wrong place.
THE ONE DIAGNOSTIC
Query your authoritative nameservers directly. It separates "wrong" from "waiting" in thirty seconds.
THE ONE PREVENTION
Export your zone and note where DNS lives.
