Remove pdns with 'yum remove pdns*'. It's possible pdns conflict with other application. If using pdns as dns server, change to other (bind/nsd/djbdns).
I have solve the problems as below:
/etc/yum.repos.d/mratwork.repo file and change:
mirrorlist=http://mirrors.fedoraproject.org/mirrorlist?repo=epel-5&arch=$basearch
to
mirrorlist=http://mirrors.fedoraproject.org/mirrorlist?repo=epel-6&arch=$basearch
Then run yum clean all and then try and install qmail-toaster again.
NOTE: you may have an issue with the GPG key being different, you need to grab the EPEL6 one or just use:
yum install qmail-toaster --nogpgcheck