Where to start.
WHAT TO ESTABLISH FIRST
Is it all mail or some? All recipients or specific providers? Incoming or outgoing? When did it start?
WHY THOSE QUESTIONS
They point at completely different causes.
ALL OUTGOING MAIL FAILING
Authentication, blacklisting, or a server problem.
MAIL TO ONE PROVIDER FAILING
That provider's filtering, or a reputation issue with them.
MAIL FROM ONE SOURCE FAILING
That sending source is not authenticated.
INCOMING MAIL NOT ARRIVING
MX records, routing, or a full mailbox.
WHAT TO CHECK IN ORDER
cPanel Email Deliverability, for authentication Blacklist status Whether the message was accepted or rejected The mail logs
WHAT THE LOGS SHOW
Whether a message was accepted, deferred or rejected, and why.
That is the definitive answer.
WHAT TO ASK US
Open a ticket with the exact error and a sample recipient address.
