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 doesnt 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