ovirt in version 4.2 choose to incorporate postfix as a mandatory MTA:

yum erase postfix
...
Removing:
 postfix                                     x86_64                   2:2.10.1-6.el7                    @base                         12 M
Removing for dependencies:
 cockpit-ovirt-dashboard                     noarch                   0.11.28-1.el7                     @ovirt-4.2                    15 M
 ovirt-host                                  x86_64                   4.2.3-1.el7                       @ovirt-4.2                    11 k
 ovirt-hosted-engine-setup                   noarch                   2.2.22.1-1.el7                    @ovirt-4.2                   2.2 M

Is there a way to change that ? It's not about postfix being inferior or superior to other solutions. It's that it didn't ask any thing, didn't check if one was already installed. It's just installed.

For example:
rpm -q --provides postfix
MTA
config(postfix) = 2:2.10.1-6.el7
postfix = 2:2.10.1-6.el7
postfix(x86-64) = 2:2.10.1-6.el7
server(smtp)
smtpd
smtpdaemon

rpm -q --provides sendmail
MTA
config(sendmail) = 8.14.7-5.el7
sendmail = 8.14.7-5.el7
sendmail(x86-64) = 8.14.7-5.el7
server(smtp)
smtpdaemon

There is a lot of other dependencies to declare other than postfix, MTA would have been better.