virtual users, mailer daemon send mails to non existant recipient and dovecot store it

[Sam]

Hello,

Sometime when we receive a spam or virus that is detected as it, mailer 
daemon send a reply to the sender to inform that the message is a spam 
or content viruses.

The problem is that the sender of the spam as something like 
voicemail at ourdomain.fr ( the user voicemail doesn't exist in our database )

And sometimes dovecot create the directory and store the reply 's mail...


Aug 23 16:07:31 mail3 postfix/cleanup[15687]: C7EEB406FFFD: 
message-id=<20160823140731.C7EEB406FFFD at mail3.ourdomain.fr>
Aug 23 16:07:31 mail3 postfix/qmgr[12987]: C7EEB406FFFD: from=<>, 
size=14280, nrcpt=1 (queue active)
Aug 23 16:07:31 mail3 postfix/bounce[15800]: 824D7406FFFC: sender 
non-delivery notification: C7EEB406FFFD
Aug 23 16:07:31 mail3 postfix/qmgr[12987]: 824D7406FFFC: removed
Aug 23 16:07:31 mail3 dovecot: auth: Debug: master in: 
USER#0111#011voicemail#011service=lda
Aug 23 16:07:31 mail3 dovecot: auth: Debug: userdb out: 
USER#0111#011voicemail#011uid=1001#011gid=1001#011home=/home/vmail/voicemail
Aug 23 16:07:31 mail3 dovecot: lda(voicemail): 
msgid=<20160823140731.C7EEB406FFFD at mail3.ourdomain.fr>: saved mail to INBOX
Aug 23 16:07:31 mail3 postfix/pipe[15791]: C7EEB406FFFD: 
to=<voicemail at ourdomain.fr>, relay=dovecot, delay=0.02, 
delays=0/0/0/0.01, dsn=2.0.0, status=sent (delivered via dovecot service)
Aug 23 16:07:31 mail3 postfix/qmgr[12987]: C7EEB406FFFD: removed

here is the stored mail :

Return-Path: <MAILER-DAEMON>
Delivered-To: voicemail at ourdomain.fr
Received: by mail3.ourdomain.fr (Postfix)
         id C7EEB406FFFD; Tue, 23 Aug 2016 16:07:31 +0200 (CEST)
Date: Tue, 23 Aug 2016 16:07:31 +0200 (CEST)
From: MAILER-DAEMON at ourdomain.fr (Mail Delivery System)
Subject: Undelivered Mail Returned to Sender
To: voicemail at ourdomain.fr
Auto-Submitted: auto-replied
MIME-Version: 1.0
Content-Type: multipart/report; report-type=delivery-status;
         boundary="824D7406FFFC.1471961251/mail3.ourdomain.fr"
Message-Id: <20160823140731.C7EEB406FFFD at mail3.ourdomain.fr>

This is a MIME-encapsulated message.

--824D7406FFFC.1471961251/mail3.ourdomain.fr
Content-Description: Notification
Content-Type: text/plain; charset=us-ascii

This is the mail system at host mail3.ourdomain.fr.

I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.

For further assistance, please send mail to postmaster.

If you do so, please include this problem report. You can
delete your own text from the attached returned message.

                    The mail system

<existing.user at ourdomain.com> (expanded from
     <existing.user at ourdomain.fr>): host
     mails.collaboration-sfr.com[86.64.240.34] said: 552 5.2.0
     <ae7X1t00115ZlG601e7Xvw> reject for policy reason : spam detected 
in your
     mail (in reply to end of DATA command)

--824D7406FFFC.1471961251/mail3.ourdomain.fr
Content-Description: Delivery report
Content-Type: message/delivery-status

Reporting-MTA: dns; mail3.ourdomain.fr
X-Postfix-Queue-ID: 824D7406FFFC
X-Postfix-Sender: rfc822; voicemail at ourdomain.fr
Arrival-Date: Tue, 23 Aug 2016 16:07:29 +0200 (CEST)

Final-Recipient: rfc822; existing.user at ourdomain.com
Original-Recipient: rfc822;existing.user at ourdomain.fr
Action: failed
Status: 5.2.0
Remote-MTA: dns; mails.collaboration-sfr.com
Diagnostic-Code: smtp; 552 5.2.0 <ae7X1t00115ZlG601e7Xvw> reject for policy
     reason : spam detected in your mail

--824D7406FFFC.1471961251/mail3.ourdomain.fr
Content-Description: Undelivered Message
Content-Type: message/rfc822

Return-Path: <voicemail at ourdomain.fr>
Received: from 177.222.108.254.dynamic.on.com.br (unknown [177.222.108.254])
         by mail3.ourdomain.fr (Postfix) with ESMTP id 824D7406FFFC
         for <existing.user at ourdomain.fr>; Tue, 23 Aug 2016 16:07:29 
+0200 (CEST)
From:voicemail at ourdomain.fr
To:existing.user at ourdomain.fr
Subject: [Vigor2820 Series] New voice mail message from 01425939048 on 
2016/08/23 11:07:28
MIME-Version: 1.0
Content-Type: multipart/mixed; boundary="5A1b791c537d41f1"


--5A1b791c537d41f1
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline

Dear existing.user :
         There is a message for you from 01425939048, on 2016/08/23 
11:07:28 .
         You might want to check it when you get a chance.Thanks!



--5A1b791c537d41f1
Content-Type: audio/x-wav; name="Message_from_01425939048.wav.zip"
Content-Transfer-Encoding: BASE64
Content-Description: Voicemail sound attachment.
Content-Disposition: attachment; filename="Message_from_01425939048.wav.zip"

UEsDBBQAAAAIAGZiF0n9ycl98x4AAE54AAAQAAAAODU5MjE2MjE1MDA4LndzZuxbW2/jWHJ+
dgP9H9hCMJbaHrWuljW2e2FJlCz1iLJk3T39QJG0SIkXNS/WpcdAZgYJctmXBNgE+7jAArkB
uTzkKUD+jLFJ9l+kDnl4Ubd4KM/0dIBgG6ZskfVV1amqU6fqHPb5L1aK/IvXz5+dL1huzk4F
9OdMm1ASf3HYuCq9YZqHr891SzUlBT17/ozC/855weB0aWFKmvo6cPvV9n3nyfkrl8W585CS
WXVqgTyQcmPfATEvbsuVy+7lLULdmLqkTpMLXTM1c70QkhPBMDVZ40SVFQW9X+85P6ZpavCP
...

I don't understand why I don't have the same behavior that when I send a 
mail to a non existant address ( <tottttt at ourdomain.fr>: Recipient 
address rejected: User unknown in virtual mailbox table )

How can I fix it?

Thanks a lot!

Samuel