Some incoming emails bounce in an endless loop

I noticed a lot of activity on MX when noone was sending mail to the server. The following lines keep being thrown at the log repeatedly, non-stop:

2015-04-13T15:59:33.902084+00:00 chipmonk leap-mx: [-] Processing new mail at '/var/mail/vmail/Maildir/new/1428940773.Vfe01I615cdM793805.chipmonk'
2015-04-13T15:59:34.476532+00:00 chipmonk leap-mx: [-] Mail owner: bounce
2015-04-13T15:59:34.477137+00:00 chipmonk leap-mx: [-] Starting factory 
2015-04-13T15:59:34.478709+00:00 chipmonk leap-mx: [LEAPPostfixTCPMapAliasServer,843,0.0.0.0] Query key: bounce@chipmonk.cdev.bitmask.net
2015-04-13T15:59:34.478983+00:00 chipmonk leap-mx: [LEAPPostfixTCPMapAliasServer,843,0.0.0.0] Starting factory 
2015-04-13T15:59:34.494935+00:00 chipmonk leap-mx: [HTTP11ClientProtocol,client] No PGP public key found for bounce.
2015-04-13T15:59:34.495058+00:00 chipmonk leap-mx: [HTTP11ClientProtocol,client] No public key, stopping the processing chain
2015-04-13T15:59:34.822545+00:00 chipmonk leap-mx: [HTTP11ClientProtocol,client] Stopping factory 
2015-04-13T15:59:34.823055+00:00 chipmonk leap-mx: [-] Removing '/var/mail/vmail/Maildir/new/1428940772.Vfe01I5fa33M516689.chipmonk'
2015-04-13T15:59:34.823244+00:00 chipmonk leap-mx: [-] Done removing
2015-04-13T15:59:34.825150+00:00 chipmonk leap-mx: [HTTP11ClientProtocol,client] Stopping factory 

The incoming mail directory keeps receiving messages like this one:

Return-Path: 
X-Original-To: bounce@chipmonk.cdev.bitmask.net
Delivered-To: bounce@chipmonk.cdev.bitmask.net
Received: by chipmonk.cdev.bitmask.net (Postfix, from userid 0)
        id 0D34E613BD; Mon, 13 Apr 2015 16:02:41 +0000 (UTC)
Content-Type: multipart/mixed; boundary="===============2899879751140625159=="
MIME-Version: 1.0
From: bounce@chipmonk.cdev.bitmask.net
To: bounce@chipmonk.cdev.bitmask.net
Date: Mon, 13 Apr 2015 16:02:39 +0000
Subject: Delivery failure
Message-Id: <20150413160251.0D34E613BD@chipmonk.cdev.bitmask.net>

--===============2899879751140625159==
Content-Type: text/plain; charset="us-ascii"
MIME-Version: 1.0
Content-Transfer-Encoding: 7bit

Delivery to the following recipient failed:
    bounce@chipmonk.cdev.bitmask.net

Reasons:
    Missing PubKey: There was a problem locating the user's public key in our database.

Original message:
    Return-Path: 
X-Original-To: bounce@chipmonk.cdev.bitmask.net
Delivered-To: bounce@chipmonk.cdev.bitmask.net
Received: by chipmonk.cdev.bitmask.net (Postfix, from userid 0)
 id D0A705FBFE; Mon, 13 Apr 2015 16:02:00 +0000 (UTC)
Content-Type: multipart/mixed; boundary="===============5050909976221440802=="
MIME-Version: 1.0
From: bounce@chipmonk.cdev.bitmask.net
To: bounce@chipmonk.cdev.bitmask.net
Date: Mon, 13 Apr 2015 16:01:59 +0000
Subject: Delivery failure
Message-Id: <20150413160237.D0A705FBFE@chipmonk.cdev.bitmask.net>

--===============5050909976221440802==
Content-Type: text/plain; charset="us-ascii"
MIME-Version: 1.0
Content-Transfer-Encoding: 7bit

Delivery to the following recipient failed:
    bounce@chipmonk.cdev.bitmask.net

Reasons:
    Missing PubKey: There was a problem locating the user's public key in our database.

Original message:
    Return-Path: 
X-Original-To: bounce@chipmonk.cdev.bitmask.net
(...)

Notice that the "original message" is repeated many many times, and currently each "complete" message has 5.4 MB. New messages keep being written to the maildir, being processed (outputting the mx log lines as noted above), removed and bounced again.

A sequence of two directory listings outputs different messages:

root@chipmonk:~# ls -alh /var/mail/vmail/Maildir/new/
total 11M
drwx------ 2 vmail vmail 4.0K Apr 13 16:04 .
drwx------ 5 vmail vmail 4.0K Mar 23 16:51 ..
-rw------- 1 vmail vmail 5.4M Apr 13 16:04 1428941068.Vfe01I61665M565542.chipmonk
-rw------- 1 vmail vmail 5.4M Apr 13 16:04 1428941069.Vfe01I612afM663472.chipmonk
root@chipmonk:~# ls -alh /var/mail/vmail/Maildir/new/
total 11M
drwx------ 2 vmail vmail 4.0K Apr 13 16:04 .
drwx------ 5 vmail vmail 4.0K Mar 23 16:51 ..
-rw------- 1 vmail vmail 5.4M Apr 13 16:04 1428941069.Vfe01I612afM663472.chipmonk
-rw------- 1 vmail vmail 5.4M Apr 13 16:04 1428941070.Vfe01I6163eM499865.chipmonk
root@chipmonk:~# 

For some reason, a bounced message is bouncing again and again.

(from redmine: created on 2015-04-13, closed on 2015-04-21, relates #6854 (closed), relates #5852 (closed))