as I read on the official rfc2822
The "Message-ID:" field provides a unique message identifier that
refers to a particular version of a particular message. The
uniqueness of the message identifier is guaranteed by the host that
generates it (see below). This message identifier is intended to be
machine readable and not necessarily meaningful to humans. A message
identifier pertains to exactly one instantiation of a particular
message; subsequent revisions to the message each receive new message
identifiers.
Note: There are many instances when messages are "changed", but those
changes do not constitute a new instantiation of that message, and
therefore the message would not get a new message identifier. For
example, when messages are introduced into the transport system, they
are often prepended with additional header fields such as trace
fields (described in section 3.6.7) and resent fields (described in
section 3.6.6). The addition of such header fields does not change
the identity of the message and therefore the original "Message-ID:"
field is retained. In all cases, it is the meaning that the sender
of the message wishes to convey (i.e., whether this is the same
message or a different message) that determines whether or not the
"Message-ID:" field changes, not any particular syntactic difference
that appears (or does not appear) in the message.
An email message can hold the same ID in some particular cases. For example, take a look at the two following message header:
Return-Path: <xxx.xxx@xxx.it>
Received: from relaypsm.eng.it (xxxx.xxx.it [xxx.xxx.xxx.xxx])
by xxxxx-xxx.xxxxx.it (8.14.4/8.14.4) with ESMTP id v0PJ6wOD014924
(version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=FAIL)
for <xxxxx@xxx.xxxxx.it>; Wed, 25 Jan 2017 20:06:59 +0100
X-Icontrol: Sent by Inrete Icontrol
Received: from MailAV (unknown [xxx.xxx.xxx.xxx])
by deliver (Postfix) with ESMTP id 52B743A4
for <xxxxx@xxx.xxxxx.it>; Wed, 25 Jan 2017 20:06:58 +0100 (CET)
Received: from LBURRINIW (unknown [192.168.63.9])
by xxxxx.xxx.it (Postfix) with ESMTPSA id 8D80339F
for <xxxxx@xxx.xxxxx.it>; Wed, 25 Jan 2017 20:06:51 +0100 (CET)
From: "Luca Burrini" <luca.burrini@eng.it>
To: <xxxxx@xxx.xxxxx.it>
References:
In-Reply-To:
Subject: fatture xxxxx
Date: Wed, 25 Jan 2017 20:06:51 +0100
Message-ID: <!&!AAAAAAAAAAAYAAAAAAAAAHCgtCXXdtBNl+uXnP8eDHPCgAAAEAAAAGutlHZJGkpAhmPV6f+ofh8BAAAAAA==@eng.it>
MIME-Version: 1.0
Content-Type: multipart/mixed;
boundary="----=_NextPart_000_00D6_01D27746.91533090"
X-Mailer: Microsoft Office Outlook 12.0
Thread-Index: AdCOWOCYZlK03GM4S5Cd5xo9GuRjGAAorxQwBlBUrvAFQ/QDgAew8EzACjCMyCAGU/qYMAV4vr+QBuilgWAKzjrI4AagZVDwBqrsgiAABfH4YAXsTykQBafYYmAEIBNvwBSv7j/wBgnvoHAB0wKvkAWE5BHA
Content-Language: it
X-KLMS-Rule-ID: 1
X-KLMS-Message-Action: clean
X-KLMS-AntiSpam-Status: not scanned, disabled by settings
X-KLMS-AntiPhishing: not scanned, disabled by settings
X-KLMS-AntiVirus: Kaspersky Security 8.0 for Linux Mail Server, version 8.0.1.721, bases: 2017/01/25 14:07:00 #8842754
X-KLMS-AntiVirus-Status: Clean, skipped
X-IControl-Milter-Checked: yes (IControlServer: xxxxx-NEW SrcIpType: UnTrusted SrcIpHeaderType: Undefined Authenticated: no)
X-IControl-Milter-SPF-Checked: yes (IControlServer: CEDACRIPEC1-NEW HeloSPFType: none FromSPFType: pass HeloHeaderSPFType: Undefined FromHeaderSPFType: pass)
X-IControl-Milter-MD5SUM: c803ab2c6498f91967e8cb2f5954f43a
and
Return-Path: <xxx.xxx@xxx.it>
Received: from xxxxx.xxx.it (xxxxx.xxx.it [xxx.xxx.xxx.xxx])
by xxxxx-new.xxxxx.it (8.14.4/8.14.4) with ESMTP id uBSGqOAP023446
(version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=FAIL)
for <xxxxx@xxx.xxxxx.it>; Wed, 28 Dec 2016 17:52:24 +0100
X-Icontrol: Sent by Inrete Icontrol
Received: from MailAV (unknown [161.27.15.23])
by deliver (Postfix) with ESMTP id A0FD33AC
for <xxxxx@xxx.xxxxx.it>; Wed, 28 Dec 2016 17:52:23 +0100 (CET)
Received: from LBURRINIW (unknown [xxx.xxx.xxx.xxx])
by relaypsm.eng.it (Postfix) with ESMTPSA id 7D2D83CC
for <xxxxx@xxx.xxxxx.it>; Wed, 28 Dec 2016 17:52:17 +0100 (CET)
From: "Luca Burrini" <xxx.xxxxx@xxx.it>
To: <xxxxx@xxx.xxxxx.it>
References:
In-Reply-To:
Subject: fatture xxx
Date: Wed, 28 Dec 2016 17:52:16 +0100
Message-ID: <!&!AAAAAAAAAAAYAAAAAAAAAHCgtCXXdtBNl+uXnP8eDHPCgAAAEAAAAGutlHZJGkpAhmPV6f+ofh8BAAAAAA==@eng.it>
MIME-Version: 1.0
Content-Type: multipart/mixed;
boundary="----=_NextPart_000_0092_01D26133.2135B540"
X-Mailer: Microsoft Office Outlook 12.0
Thread-Index: AdCOWOCYZlK03GM4S5Cd5xo9GuRjGAAorxQwBlBUrvAFQ/QDgAew8EzACjCMyCAGU/qYMAV4vr+QBuilgWAKzjrI4AagZVDwBqrsgiAABfH4YAXsTykQBafYYmAEIBNvwBSv7j/wBgnvoHAB0wKvkA==
Content-Language: it
X-KLMS-Rule-ID: 1
X-KLMS-Message-Action: clean
X-KLMS-AntiSpam-Status: not scanned, disabled by settings
X-KLMS-AntiPhishing: not scanned, disabled by settings
X-KLMS-AntiVirus: Kaspersky Security 8.0 for Linux Mail Server, version 8.0.1.721, bases: 2016/12/28 09:52:00 #8723401
X-KLMS-AntiVirus-Status: Clean, skipped
X-IControl-Milter-Checked: yes (IControlServer: CEDACRIPEC1-NEW SrcIpType: UnTrusted SrcIpHeaderType: Undefined Authenticated: no)
X-IControl-Milter-SPF-Checked: yes (IControlServer: CEDACRIPEC1-NEW HeloSPFType: none FromSPFType: pass HeloHeaderSPFType: Undefined FromHeaderSPFType: pass)
X-IControl-Milter-MD5SUM: 30378f3a90c4e4051ecd9eab79feaa02
They hold the same ID but the two emails, trust me, belong to different transmissions. In addition they hold different attachments.
Can you tell me why the email IDs are the same?
You can get different messages with the same message-Id. Here's how: