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.