Since CentOS 5.9 the postfix package requires the package "mysql".
With the vanilla CentOS mysql packages the "mysql" package is the client.
This means that you only need to install the mysql client to use postfix.
However when using MariaDB or Oracle RPMs the MariaDB-server package provides "mysql".
This means that one must install MariaDB-server to install postfix, whereas not all servers need MariaDB-server, some just need MariaDB-client.