The error
stat=Deferred: 454 4.7.1 <*>: Relay access denied
was found in /var/log/maillog on the ZPanel linux server.

The emails sent from the server was not arriving.

Issue was fixed by editing file:
vi /etc/postfix/main.cf
then under "# network settings" section, you see line:
mynetworks = all
enter editting mode by hittin "a" key.

append ",127.0.0.1" to that line so it is:
mynetworks = all,127.0.0.1
then save changes in the file and quit editor: Ctrl + C ; :wq ; Entrer key

then i restarted postfix: service postfix restart
and when email was sent it arrived successfully to the destination mailbox.