Support for message/global-delivery-status (RFC 6533) missing

[SirDariusz]

Describe the bug
The content type "message/global-delivery-status" (RFC 6533, "Internationalized Delivery Status and Disposition Notifications", see https://www.iana.org/go/rfc6533) is not recognized as a DSN by the parser. The parser creates a normal "MimePart" instead of a "MessageDeliveryStatus".

Platform:

• OS: Windows 10
• .NET Runtime: CoreCLR
• .NET Framework: NetCore 2.1
• MimeKit Version: 2.3.2

To Reproduce

1. Create MimeMessage from content:

Return-Path: <>
X-Original-To: REDACTED@REDACTED
Delivered-To: REDACTED@REDACTED
Received: by REDACTED (Postfix)
	id 3C0C742807; Sun, 27 Oct 2019 19:41:57 +0100 (CET)
Date: Sun, 27 Oct 2019 19:41:57 +0100 (CET)
From: MAILER-DAEMON@REDACTED (Mail Delivery System)
Subject: Undelivered Mail Returned to Sender
To: REDACTED@REDACTED
Auto-Submitted: auto-replied
MIME-Version: 1.0
Content-Type: multipart/report; report-type=delivery-status;
	boundary="5B4C54275B.1572201717/REDACTED"
Content-Transfer-Encoding: 8bit
Message-Id: <20191027184157.3C0C742807@REDACTED>

This is a MIME-encapsulated message.

--5B4C54275B.1572201717/REDACTED
Content-Description: Notification
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 8bit

This is the mail system at host REDACTED.

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

<REDACTED@REDACTED>: SMTPUTF8 is required, but was not offered by host
    REDACTED[REDACTED]

--5B4C54275B.1572201717/REDACTED
Content-Description: Delivery report
Content-Type: message/global-delivery-status
Content-Transfer-Encoding: 8bit

Reporting-MTA: dns; REDACTED
X-Postfix-Queue-ID: 5B4C54275B
X-Postfix-Sender: rfc822; REDACTED@REDACTED
Arrival-Date: Sun, 27 Oct 2019 19:41:56 +0100 (CET)

Final-Recipient: rfc822; REDACTED
Original-Recipient: rfc822;REDACTED
Action: failed
Status: 5.6.7
Diagnostic-Code: X-Postfix; SMTPUTF8 is required, but was not offered by host
    REDACTED[REDACTED]

--5B4C54275B.1572201717/REDACTED
Content-Description: Undelivered Message
Content-Type: message/global
Content-Transfer-Encoding: 8bit

Return-Path: <REDACTED@REDACTED>
Received: from REDACTED (unknown [REDACTED])
	by REDACTED (Postfix) with UTF8SMTP id 5B4C54275B
	for <REDACTED@REDACTED>; Sun, 27 Oct 2019 19:41:56 +0100 (CET)
X-SN-MUUID: CE20727C-B201-4CE3-8639-C7418A9F789D
MIME-Version: 1.0
From: REDACTED@REDACTED
To: REDACTED@REDACTED
Date: 27 Oct 2019 19:42:41 +0100
Subject: REDACTED
Content-Type: text/html; charset=utf-8
Content-Transfer-Encoding: base64

BASE64_CONTENT_REDACTED


--5B4C54275B.1572201717/REDACTED--

2. Second MimePart is type "MimePart" instead of "MessageDeliveryStatus"

Expected behavior
Second MimePart is type "MessageDeliveryStatus"

[jstedfast]

Thanks.

[jstedfast]

MimeKit 2.4.0 has just been released with this feature.