Mail being mis-delivered?

Question

I'm seeing a fair number of emails being delivered to accounts that they aren't directly addressed to, almost as if it was being mis-delivered. Of course, they have all be spam mails so it's not a major problem, just more of the "why is that happening, and can I eliminate it" issues.
First hunch is that email addresses have been BCC'd to the original and I'm seeing the result of that, but I don't know if there's any way to discern if that's the case from looking at anything in the headers or logs somewhere. The one consistent element that I've noticed is that all the mails are not addressed with a name preceding the address in the To: field, just "<[email protected]>"
Any input appreciated!
-Doug
Sample email header:
Return-Path: <[email protected]>
Received: from murder ([unix socket])
by server.mydomain.com (Cyrus v2.3.8-OS X Server 10.5: 9G7013y) with LMTPA;
Thu, 16 Dec 2010 11:43:59 -0800
X-Sieve: CMU Sieve 2.3
Received: from localhost (localhost [127.0.0.1])
by server.mydomain.com (Postfix) with ESMTP id 2CA1155AE78;
Thu, 16 Dec 2010 11:43:57 -0800 (PST)
X-Quarantine-ID: <fDHrH1xpu393>
X-Virus-Scanned: amavisd-new at mydomain.com
X-Spam-Flag: YES
X-Spam-Score: 17.833
X-Spam-Level: ***************
X-Spam-Status: Yes, score=17.833 tagged_above=2 required=6
tests=[BAYES_99=3.5, HTML_MESSAGE=0.001, RCVDIN_BL_SPAMCOPNET=1.96,
RCVDINPBL=0.905, RCVDINXBL=3.033, RDNS_NONE=0.1,
URIBLABSURBL=1.86, URIBLJPSURBL=1.501, URIBLOBSURBL=1.5,
URIBL_SBL=1.499, URIBLSCSURBL=0.474, URIBLWSSURBL=1.5]
Received: from server.mydomain.com ([127.0.0.1])
by localhost (server.mydomain.com [127.0.0.1]) (amavisd-new, port 10024)
with ESMTP id fDHrH1xpu393; Thu, 16 Dec 2010 11:43:56 -0800 (PST)
Received: from cbuwo (unknown [217.118.93.89])
by server.mydomain.com (Postfix) with ESMTP id A5E2755AE64;
Thu, 16 Dec 2010 11:43:52 -0800 (PST)
X-Sender: <[email protected]>
Date: Thu, 16 Dec 2010 13:36:23 -0700
MIME-Version: 1.0
Reply-To: "Janna Henriette" <[email protected]>
In-Reply-To: <e3b901cb9916$8479d1dd$56d1f2c7@koscb51>
Message-ID: <[email protected]>
To: <[email protected]>
Sender: <[email protected]>
Subject: *JUNK MAIL* What Else Do The Spammers Try to Sell??
From: "Janna Henriette" <[email protected]>
Content-Type: multipart/alternative;
boundary="----=Part_11585_0611B466499B.B7AECD1F"
User-Agent: Mozilla/5.038 (Windows; U; NT4.0; U; NT4.0; en-us) Gecko/25250101
Message was edited by: Douggo

Answer 1

In case someone else finds this and has the same question I had, it was accepting mail because I had ACL set up for mail in Server Admin. As the Mac server docs say, mail service uses ACL differently from any of the other services, and if you set up any ACL for mail, then the Mail tab in Workgroup Manager is effectively ignored.
Counterintuitively, setting the Mail ACL in Server Admin to 'Allow all users and groups' allows mail to not be allowed via the Workgroup manager.