We've seen this on two different platforms (with different admins, one OSE and one multi-servers NE).

Since 8.0.3, the admin mailbox gets a copy of all quarantined spams (markeds as spam by spamassassin).

Actually the problem is two folds.
First, the spam should not be quarantined (they're spams, not virii and amavis is setup to deliver all spams to end users).
Second, the admin account get this copy and there nothing delivered to the spam account (while when a user marks a spam, it is correctly copied to spam account's inbox).

The sender is "Content-type at zimbra-mta.domain.tld" (replace with real server name) and look like this:
Code:
Content type: Spam
Internal reference code for the message is 08654-07/zsatBFY44RuA

First upstream SMTP client IP address: [127.0.0.1] localhost.localdomain
According to a 'Received:' trace, the message apparently originated at:
  [162.105.243.14], localhost localhost.localdomain [127.0.0.1]

Return-Path: <xcrfzrbtse@docomo.ne.jp>
From: <smfkopcvgb@docomo.ne.jp>
Message-ID: <20130526165245.10AE573@zimbra-mta.domain.tld>
Subject: {Spam?}
  =?ISO-2022-JP?B?GyRCJV8lSyVtJUgkPyRDJD8jNTh9JEdFdkEqOzBLZiEqIVo1ZjZLJE4hSU4iIUlJLExZMXw1QSFbP2skSzh4MyshKhsoQg==?=
The message has been quarantined as: spam-zsatBFY44RuA.gz

The message WILL BE relayed to:
<user@domain.tld>

Spam scanner report:
Spam detection software, running on the system "zimbra-mta.domain.tld",
has identified this incoming email as possible spam.  The original
message has been attached to this so you can view it or label
similar future email.  If you have any questions, see
@@CONTACT_ADDRESS@@ for details.

Content preview:  ////  ƒ~ƒjƒƒg”— ””“`  ////                           
   * „  „ャ–。・'*:.„Ÿ„Ÿ„Ÿ„Ÿ„Ÿ„Ÿ„Ÿ„Ÿ„Ÿ„Ÿ„Ÿ„Ÿ„Ÿ„Ÿ„** „ 人 „                    „
   人  ̄‚x ̄„ ‚ ‚‚‚ƒMƒƒƒ“ƒuƒ‹‚ŽŽ‚オ’…‚「‚ス–@‘・… „  ̄‚x ̄  „ * „                    „
   „ * „ッ„Ÿ„Ÿ„Ÿ„Ÿ„Ÿ„Ÿ„Ÿ„Ÿ„Ÿ„Ÿ„Ÿ„Ÿ„Ÿ。.'゜*:・–„ョ [...] 

Content analysis details:   (19.5 points, 5.0 required)

 pts rule name              description
---- ---------------------- --------------------------------------------------
 1.6 URIBL_WS_SURBL         Contains an URL listed in the WS SURBL blocklist
                            [URIs: loto-x.info]
 1.2 URIBL_JP_SURBL         Contains an URL listed in the JP SURBL blocklist
                            [URIs: loto-x.info]
 0.6 URIBL_SC_SURBL         Contains an URL listed in the SC SURBL blocklist
                            [URIs: loto-x.info]
-1.0 ALL_TRUSTED            Passed through trusted hosts only via SMTP
 3.5 BAYES_99               BODY: Bayes spam probability is 99 to 100%
                            [score: 1.0000]
 0.0 T_LONG_HEADER_LINE_80  A header line contains 80-159 characters
 1.0 HK_RANDOM_FROM         From username looks random
 0.0 HK_RANDOM_ENVFROM      Envelope sender username looks random
 2.3 FSL_HELO_BARE_IP_1     FSL_HELO_BARE_IP_1
 0.0 T_FSL_HELO_NON_FQDN_2  T_FSL_HELO_NON_FQDN_2
 0.0 T_FSL_HELO_BARE_IP_2   T_FSL_HELO_BARE_IP_2
 6.0 SPAM_MC                Spam detecte par le Cluster MailCleaner
 0.1 URIBL_SBL_A            Contains URL's A record listed in the SBL blocklist
                            [URIs: loto-x.info]
 1.7 URIBL_BLACK            Contains an URL listed in the URIBL blacklist
                            [URIs: loto-x.info]
 1.6 URIBL_SBL              Contains an URL listed in the SBL blocklist
                            [URIs: loto-x.info]
 0.8 RDNS_NONE              Delivered to internal network by a host with no rDNS
-0.0 T_NOT_A_PERSON         List, replier, bot, etc.  Filters: skip auto-reply

Return-Path: <xcrfzrbtse@docomo.ne.jp>
Received: from localhost (localhost.localdomain [127.0.0.1])
        by zimbra-mta.domain.tld (Postfix) with ESMTP id 396FB83
        for <user@domain.tld>; Sun, 26 May 2013 18:52:45 +0200 (CEST)
X-Virus-Scanned: amavisd-new at admin-domain.tld
Received: from zimbra-mta.domain.tld ([127.0.0.1])
        by localhost (zimbra-mta.domain.tld [127.0.0.1]) (amavisd-new, port 10026)
        with ESMTP id VQGrtwDrMpRA for <user@domain.tld>;
        Sun, 26 May 2013 18:52:45 +0200 (CEST)
Received: from domain.tld (external-mta-relay [192.168.101.118])
        by zimbra-mta.domain.tld (Postfix) with ESMTPS id 10AE573
        for <dd.34@domain.tld>; Sun, 26 May 2013 18:52:45 +0200 (CEST)
Received: from [162.105.243.14] (helo=62.93.225.70)
        by domain.tld stage1 with smtp 
        (Exim MailCleaner) 
        id 1UgeBX-0003kV-Dw 
        for <user@domain.tld> 
        from <xcrfzrbtse@docomo.ne.jp>; Sun, 26 May 2013 18:52:44 +0200
X-MailCleaner-SPF: softfail
From: <smfkopcvgb@docomo.ne.jp>
To: user@domain.tld
Date: Mon, 27 May 2013 01:52:40 +0900
X-Info:user@domain.tld
MIME-Version: 1.0
Content-Type: text/plain; charset="Shift_JIS"
List-Id: 8
X-NiceBayes: disabled (no database ?)
X-PreRBLs: is spam (SPAMHAUS)
X-MailCleaner-Information: Please contact  for more information
X-MailCleaner-ID: 1UgeBY-0003kZ-7u
X-MailCleaner: Found to be clean
X-MailCleaner-SpamCheck: spam, PreRBLs (SPAMHAUS)
Subject: {Spam?} =?ISO-2022-JP?B?GyRCJV8lSyVtJUgkPyRDJD8jNTh9JEdFdkEqOzBLZiEqIVo1ZjZLJE4hSU4iIUlJLExZMXw1QSFbP2skSzh4MyshKhsoQg==?=
Message-Id: <20130526165245.10AE573@zimbra-mta.domain.tld>