I don't think this should be in the 'newbie section' mainly because I have to support DBA's on many of my enterprise applications, they have many scripts that depend on the mailx command.
The mailx command generally is an offset of /bin/mail, try to issue a symlink.
ln -s /bin/mail /bin/mailx
Don't forget to start sendmail
service sendmail start
chkconfig sendmail on
Now issue your command:
mailx -s "test" firstname.lastname@example.org