Symptoms
When a cPanel account has the default/catch-all email address set as the cPanel username and Exim reports
cpuser@server.hostname.tld is undeliverable: The mail server could not deliver mail to cpuser@server.hostname.tld . The account or domain may not exist, they may be blacklisted, or missing the proper dns entries.
and the account never receives the email.
Description
This issue occurs because Exim doesn't have a configuration set for the hostname and should exist in /etc/localdomains
Workaround
If this hostname doesn't exist in /etc/localdomains it can easily be added with
echo $(hostname) >> /etc/localdomains