In order to have mail working properly on APEX, mail must be setup properly.
This note has a lot of information about how I modified linux systems to work with postfix properly.
Configuring mail properly on a linux server is critical for APEX to send mail properly.
Configuring Postfix as a Gmail Relay on CentOS
iptables
then mail works
ora_utlmail
ora_prvtmail
/u01/app/oracle/product/12.2.0/db_1/rdbms/admin
directory for rdbms admin stuff