Sending an email to the cPanel default email address results in the email not being received or sent to a remote host.
[14:51:24 root@~]cPs# exim -bt email@example.com
router = lookuphost, transport = remote_smtp
host remoteserver.smtp.tld [x.x.x.x]
The default email address is the name of the email account associated with a cPanel's username after creating a cPanel account. If a user doesn't have email accounts configured, the emails for the account are delivered to the default account. When the server's hostname isn't configured in the "/etc/localdomains" list and custom mail routes are configured, it will cause emails delivered to the default users to arrive at the remote host instead of the local user email inbox.
Ensure the hostname domain is included in the "/etc/localdomains" file.
echo $(hostname) >> /etc/localdomains
Retest with the "exim -bt" to confirm local delivery afterward.
[15:01:16 root@~]cPs# exim -bt firstname.lastname@example.org
router = localuser, transport = dovecot_delivery_no_batch
Retry the email afterward.
Note: The transaction after testing shows the router as "localuser," which ensures the email will arrive at the inbox on the server.