For people with similar problem: I have solved it. You have to change 3 things to get sendmail working in RedHat9 as a SMARTHOST which just hands over outgoing mail to your provider.
1. In Redhat 9.0 the aliases file in /etc/aliases must be moved to /etc/mail/aliases
2. You must make some changes in the configuration, using m4.|
2a. edit /etc/mail/sendmail.mc: set define('SMARTHOST'), 'smtp.yourprovider.com')
2b. set Masquerade to your domain
3. configure sendmail using the following command: m4 /etc/mail/sendmail.mc /etc/mail/sendmail.cf
4. restart sendmail using the command: /etc/rc.d/init.d/sendmail restart
or start sendmail with the command /usr/sbin/sendmail -bd -os
5. if your system cannot find the smtp server you might also try adding your providers' nameserver to /etc/resolv.conf
If this is not the palce to be for this info, please feel free to move it to a FAQ section or so,