Sorry for the low level questions but I have two point which I need to 
better understand:
SO : SCIENTIFIC LINUX 5.5
a) yum
   In /etc/cron yum.cron correspond to the orginal.yum.cron
   so some autoupdates are enabled
   in yum.excludes are defined the packages which are not updatet
   in mine for example thereis kernel* 
   so the kernel should be not updatet

   instead an update of the kernel was done! Why?

b) installation of sendmail
   installed latest associated to SL5
   turn on the run of the service at boot (level 3 and 5)
   the sendmail deamon is started
   the configuration is the standard one 
   try to send an e-mail results dead.letter?
   Forgot somethings?