I just had the same thing with my server. Fresh Centos 6 install.
Exim would not work because "master" was bound to port 25, but postfix was not installed. After I killed the master process, I could start up exim normally.
I have another server with the same Centos 6 installation but no directadmin, and I found that postfix is installed there. It seems postfix gets removes, but the process is not stopped.
Anyway, thanks for this thread!