The Problem is, that the HELO command needs the full qualified domain
name:
MYHOST.MYDOMAIN.COM

Given the IP-Address instead of the full q.d.n it will return the
message:
can't verify that you are 100.200.300.400 .
And there is not Problem getting the IP-Address using :
get_host_address

The same message you will get with MYHOST.
Only if you submit MYHOST.MYDOMAIN.COM you will get a message like
G'day ...

Maybe it depends upon how the smtp server is set up.
Fakt is that UTL_INADDR.get_host_name() will not retrieve the full
qualified hostname.

But the Oracle Package depends on Java-Code and there ist a Class,
maybe there methods not beeing published yet in a appropiate oracle
package:
oracle.plsql.net.InternetAddress

I will have a look a this.

Greetings bernd kochs