The most common DNS casualty.
WHAT HAPPENED
Almost always one of three things.
MX RECORDS NOT RECREATED
When nameservers change, the zone does not travel. If MX records were not created at the new provider, incoming mail fails immediately.
THE MAIL HOSTNAME NOT RESOLVING
MX records point at a hostname. If that hostname has no A record at the new provider, mail cannot be delivered.
SPF OR DKIM MISSING
Outgoing mail starts failing authentication, so it lands in spam or is rejected.
Less immediately visible than incoming failure, and it damages your reputation while unnoticed.
HOW TO CHECK
Look up the MX records for your domain. Are they there, and correct? Look up the hostname they point at. Does it resolve? Look up TXT records. Is there exactly one SPF record, and a DKIM record?
Compare against cPanel > Email Deliverability.
THE FIX
Recreate the missing records at whichever provider holds your DNS.
PREVENTION
Record the full zone before any nameserver change, and recreate every record before switching.
