Your issue is caused by conflict packages from different yum repositories.
According to our installation guide, you MUST DISABLE all third-party yum repositories before installation: http://www.iredmail.org/install_iredmai … able_repos
* For CentOS or Scientific Linux, please enable CentOS/Scientific official yum repositories, and DISABLE all third-party yum repositories to avoid package conflict.
* For Red Hat Enterprise Linux, please enable Red Hat Network to install packages, or create a local yum repository with DVD/CD ISO images.