Installing Zimbra on Centos 5 -

http://www.zimbra.com/forums/installation/8726-centos-5-install-guide.html

Install Centos custom install with

Editors
Text base Internet
Development Libraries
Development Tools
Administration Tools
Base
System Tools
(you can install GUI if you want)

NOTE: things will go easier if you create yoru A records and MX records for your mail server

Make sure your /etc/hosts file has the line:
127.0.0.1 yourmailserver.yourdomain.com yourmailserver

yum remove sendmail
yum install libtool-ltdl openssl097a compat-libstdc++-296 compat-libstdc++-33 compat-db fetchmail

Edit the /etc/sudoers and comment out “defaults requiretty” (put # in front of it).
Edit the /etc/sysconfig/i18n and delete the line LANG=”en_US.UTF-8”.

cd /var/temp
wget http://files.zimbra.com/downloads/5.0.1_GA/zcs-5.0.1_GA_1902.RHEL5.20080109200629.tgz

when done
tar xzvf 5.0.1_GA/zcs-5.0.1_GA_1902.RHEL5.20080109200629.tgz
cd 5.0.1_GA/zcs-5.0.1_GA_1902.RHEL5.20080109200629

./install.sh TO INSTALL


=============

make sure to open the correct ports on your firewall

after browsing the zimbra forums, because I could only send but not recieve, I looked at
/var/log/zimbra.log
The output said that my firwall was blocking my mailserver at port 7025, after opening that port I could send and receive mail.