I ran into a problem the other day when building a new CentOS 6 VPS template. After doing some of the initial work I ran yum update, and it worked fine. Later on, after installing the AtomiCorp repo, I tried to run it again and it failed with errors about Qmail obsoleting Sendmail, not being able to remove Sendmail, and to try using the –skip-broken parameter to work around the problem.
I tried simply removing Sendmail, but it turns out that Cronie has a hardcoded dependency for it. At first it seemed pretty ridiculous to make such a core program dependent on something optional and obsolete, but after a bit of digging I found that the newer MTAs provide a sendmail interface, so all you really need to do is install Postfix or Exim and run alternatives –config mta. After that you can remove Sendmail and yum won’t complain.