SUMA: smpt address rewrite

I am trying to get email to work.
I found a post saying I had to put
java.smtp_server = ip of smtp server
in /etc/rhn/rhn.conf

I did that and the relay now gets mail sent by SUMA.
I am trying to test that with creating (and deleting and again creating) an user (who should get a welcome email).
Unfortunately the sender is root@localdomain and I cannot figure out how to configure a rewrite.

Postfix rewrite and sending email from command line is perfectly working.

Thanks for any hint.
Kind regards, Igor