Remove Old Packages
By default centos/rhel will installed sendmail as mailserver.Since we will using Postfix MTA instead of Sendmail MTA, we need to remove sendmail to make sure there is no other mailserver that will running on the same port.
Remove Sendmail Mailserver
#rpm -e sendmail --nodeps
We also need to remove cyrus-sasl default packages and we will replace cyrus-sasl with the latest version.
Remove Cyrus-SASL
#yum remove cyrus-sasl