Actually if you look at the received headers for local mail, it isn't delivered (directly) by LMTP.

Code:
Received: from zimbra.company.com (LHLO zimbra.company.com) (99.147.40.125) by
 zimbra.company.com with LMTP; Thu, 2 Sep 2010 11:28:43 -0700 (PDT)
Received: from localhost (localhost.localdomain [127.0.0.1])
	by zimbra.company.com (Postfix) with ESMTP id 5D8B92D60292
	for <user@company.com>; Thu,  2 Sep 2010 11:28:43 -0700 (PDT)
Received: from zimbra.company.com ([127.0.0.1])
	by localhost (zimbra.company.com [127.0.0.1]) (amavisd-new, port 10024)
	with ESMTP id cCDVST4Sq2Xk for <user@company.com>;
	Thu,  2 Sep 2010 11:28:43 -0700 (PDT)
Received: from zimbra.company.com (localhost.localdomain [127.0.0.1])
	by zimbra.company.com (Postfix) with ESMTP id 002322D600B6
	for <user@company.com>; Thu,  2 Sep 2010 11:28:42 -0700 (PDT)
ZWC just acts as a normal mail client.