This post describes how to set up APEX to send emails in order to allow users to subscribe to interactive report:

Configure APEX with the SMTP server, port and user credentials:
Verify the existing ACLs:

select * from dba_network_acl_privileges;

Add the APEX user, APEX_040000 in my case, to the ACL list of allowed users to send email, or in a more general form, use UDP_MAIL:

_begin
    dbms_network_acl_admin.add_privilege (
    acl       => ‘netacl.xml’,
    principal => ‘APEX__0XXXXX',
    is_grant  => TRUE,
    privilege => ‘connect’
    );
    commit;
end;
/

_begin
    dbms_network_acl_admin.add_privilege (
    acl       => ‘netacl.xml’,
    principal => ‘APEX__0XXXXX',
    is_grant  => TRUE,
    privilege => ‘resolve’
    );
    commit;
end;
/