Mail in Apex can be sen with the apex_mail package.
The most efficient approach to sending email is to create a background job (using the DBMS_JOB or DBMS_SCHEDULER package) to periodically send all mail messages stored in the active mail queue.
Luckily, this process is normally created during installation with DBMS_SCHEDULER and is named ORACLE_APEX_MAIL_QUEUE. It sends mail that is in the mail queue every 5 minutes.
To call the APEX_MAIL package from outside the context of an Application Express application, you must call apex_util.set_security_group_id.
See the APEX_INSTANCE_ADMIN Available Parameter Values that start with SMTP.
You can change them in internal