Well given this:
/usr/sbin/sendmail: No such file or directory
I would guess that you don't have sendmail installed. So try installing sendmail. They'll be a package for it in Mandriva's repositories I expect. That won't actually get you very far though unless you only plan to send email to other users on the same machine. If you want to send mail to firstname.lastname@example.org
then you'll also need to tell sendmail to pass the mail on to your ISP's mail or something. I forget exactly only dabbled briefly with sendmail ages ago. Is there a particular reason you're trying to use mailx rather than say Thunderbird or Kmail or Evolution or something else nice and GUI and easily configurable?