you should delete the current version of postfix, and, for future safety,
exclude postfix from base and updates repo /etc/yum.repo.d/CentOS-Base.repo:
Code:
[base]
name=CentOS-$releasever - Base
...
exclude=postfix*
#released updates
[updates]
name=CentOS-$releasever - Updates
...
exclude=postfix*