Require and use the puppet-alternatives module.

manifests/service.pp

@@ -12,10 +12,9 @@ class postfix::service {
     restart   => $::postfix::params::restart_cmd,
   }
   if $::osfamily == 'RedHat' {
-    exec { 'alternatives --set mta /usr/sbin/sendmail.postfix':
+    alternatives { 'mta':
+      path    => '/usr/sbin/sendmail.postfix',
       require => Service['postfix'],
-      path    => '/bin:/sbin:/usr/bin:/usr/sbin',
-      unless  => 'test /etc/alternatives/mta -ef /usr/sbin/sendmail.postfix',
     }
   }
 }

metadata.json

@@ -19,6 +19,10 @@
     {
       "name": "camptocamp/augeas",
       "version_requirement": ">=1.0.0 <2.0.0"
+    },
+    {
+      "name": "puppet/alternatives",
+      "version_requirement": ">=2.0.0 <3.0.0"
     }
   ],
   "operatingsystem_support": [