Please note this information is probably out of date.
Alternative Postfix RPMS are available from various sources:
RedHat now provide Postfix rpms as standard with both Fedora and
RedHat Enterprise Linux. Originally this was not the case and was
one of the reasons I started packaging postfix for RedHat as there were
few alternatives. However, unless you need a special configuration
not supported by RedHat out of the box you are probably
best using the Postfix rpm which comes with your RedHat distribution.