Fallback to system's sendmail if the one from Zimbra isn't installed

This will be the case if you run mailboxd and mta on different servers

zmldapsync/zmldapsync.pl

@@ -770,7 +770,7 @@ sub handle_error {
       body_str => "LDAP synchronisation for domain $domain failed at step '$step'. The error was\n$err\n",
     );
     my $transport = Email::Sender::Transport::Sendmail->new({
-      sendmail => '/opt/zimbra/common/sbin/sendmail'
+      sendmail => ( -x '/opt/zimbra/common/sbin/sendmail' ) ? '/opt/zimbra/common/sbin/sendmail' : '/usr/sbin/sendmail'
     });
     sendmail( $mail, { transport => $transport } );
   }