Put $ostype variable to sendmail::common so it's inherited to other classes.

sendmail/manifests/init.pp

@@ -28,6 +28,10 @@ class sendmail::common {
                 mode    => "0644",
                 notify  => Service["sendmail"],
             }
+            $ostype = "linux"
+        }
+        "openbsd": {
+            $ostype = "openbsd"
         }
     }
 
@@ -164,7 +168,6 @@ class sendmail::server inherits sendmail::common {
             File["/etc/sysconfig/sendmail"] {
                 content => "DAEMON=yes\nQUEUE=1h\n",
             }
-            $ostype = "linux" # For Sendmail's OSTYPE macro
         }
         default: {
             fail("sendmail::server not supported in '${::operatingsystem}'")