Postfix is eaiser to administrate and configure than Sendmail, however Sendmail  comes pre-installed on Dell EL4 systems 

Check to see if postfix is already installed

# rpm -qa postfix
postfix-2.2.10-1.RHEL4.2

if postfix is not installed use   up2date -u postfix*  to install
 
Setup postfix as the MTA rather than sendmail 

# /usr/sbin/alternatives --config mta

There are 2 programs which provide 'mta'.

  Selection    Command
-----------------------------------------------
*+ 1           /usr/sbin/sendmail.sendmail
   2           /usr/sbin/sendmail.postfix

Enter to keep the current selection[+], or type selection number: 2


Stop sendmail service if running (very important!) 

# service sendmail stop
Shutting down sendmail:                                    [  OK  ]

# chkconfig --levels 12345 sendmail off


Set postfix runlevels

# chkconfig --levels 235 postfix on
# chkconfig --list postfix
postfix         0:off   1:off   2:on    3:on    4:off   5:on    6:off

Start Postfix

# service postfix start
Starting postfix:                                          [  OK  ]


The following steps are only necessary if you encounter problems 

You may have to set the relayhost parameter in /etc/postfix/conf/main.cf if the system doesn’t route mail insitue with customer see below for example 

# relayhost is the name of the smtp server to use when sending mail
# needs to be in [] brackets other wise will think it's to route locally
relayhost = [mailhost.zen.co.uk]

If you have more trouble in getting mail to route properly insitue then make sure that DNS settings are correct for the  installation (you can test this with ping and dig)  in /etc/resolv.conf

# cat /etc/resolv.conf
; generated by /sbin/dhclient-script
nameserver 212.23.3.100
nameserver 212.23.6.100