$SUBJECT causes issues as in cases where the user/group creation fails the
issue is simply ignored.

Example:

(344/591) Installing: system-user-mail-20161020-5.1.noarch [.....done]
Additional rpm output:
Creating group mail with gid 489.
Creating user mail (Mailer daemon) with uid 489 and gid 489.
Failed to write files: Operation not permitted
warning: user mail does not exist - using root
warning: group mail does not exist - using root

And the installation continues happily after that, resulting in:

(502/591) Installing: postfix-3.2.0-4.1.x86_64 [..error]
Abort, retry, ignore? [a/r/i] (a): a
[91mInstallation of postfix-3.2.0-4.1.x86_64 failed:
Error: Subprocess failed. Error: RPM failed: usermod: group 'mail' does not
exist
error: %prein(postfix-3.2.0-4.1.x86_64) scriptlet failed, exit status 6
error: postfix-3.2.0-4.1.x86_64: install failed