ACL principal

Based on Oracle's documentation (Oracle Database 11g Release 11.2.0.3.0 - 64bit Production):

BEGIN
 DBMS_NETWORK_ACL_ADMIN.CREATE_ACL (
  acl          => 'file_name.xml', 
  description  => 'file description',
  principal    => 'user_or_role',
  is_grant     => TRUE|FALSE, 
  privilege    => 'connect|resolve',
  start_date   => null|timestamp_with_time_zone,
  end_date     => null|timestamp_with_time_zone); 
END;

If I use an user, say, SCOTT, utl_mail send out email just fine. But, If I use a role, say ADM_USERS, I will always get ACL error:

ERROR at line 1:
ORA-24247: network access denied by access control list (ACL)
ORA-06512: at "SYS.UTL_MAIL", line 654
ORA-06512: at "SYS.UTL_MAIL", line 671