All -Thanks for the responses. I tried every variation of the ACL using the hostname, localhost and the hostname of the SMTP relay server. Then one of the sys admins told me their reverse lookup was buggy,.. so I used the IP address, something I usually don't like to do and everything worked. I also specified no port range.
SQL> exec DBMS_NETWORK_ACL_ADMIN.ASSIGN_ACL('TCP_SMTP_acl.xml','<SMTP Relay Server IP address>');
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