NEW Outlook: opening EML file with "X-Unsent: 1" removes body and attachment

Question

Hi,

I'm migrating a PIA (interop) program from classic Outlook to NEW Outlook.
Current version of NEW Outlook:

• Microsoft Outlook Version: 1.2025.319.100 (Production)
• Client Version: 20250320055.21

Tests were made on Windows 11.

As NEW Outlook no longer supports COM, I save the mail properties into an EML file and open it using Pro cess.Start() (using VB.Net in Visual Studio 2022). The mail contains a subject, a body and an attachment. As the eml file also contains "X-Unsent: 1", NEW Outlook opens the mail in "new mail" mode (draft).

Tests:

• Body in plain text with X-Unsent --> body text and attachment are missing in mail
• Body in plain text without X-Unsent --> body text and attachment present, but mail not displayed as draft
• Body in HTML with X-Unsent --> body text present and attachment are missing in mail
• Body in HTML without X-Unsent --> body text and attachment present, but mail not displayed as draft

Any ideas? A bug in NEW outlook?

The same tests work correctly with classic Outlook.

@Microsoft: BTW. when I write "Pro cess.Start()" together without a space, I can't save this post. I get the message "You are not authorized to make this response. If you believe this to be in error, please refresh the page and try again."

Regards,
Patrick

Answer 1

Hi, I'm in the same boat as you with having to support applications that email out of Outlook, but I haven't been able to reproduce this problem. I'm also following along at

https://gist.github.com/SelimBasyouni/efec48a197bfefb59d4ff75797aae15e

My Outlook new is:

Microsoft Outlook Version: 1.2025.328.300 (Production)

Client Version: 20250328005.20

WebView2 Version: 134.0.3124.93 (Stable)

Session ID: b3025730-6eaf-dc4e-b651-21d5444fae01

I have opened up multiple copies of a sample.eml quoted below and I can't get it to remove the attachments, i've added and changed attachments many times to different EML files with no problem. Any modifications I make to it wind up saving it as a Draft in my Drafts folder, and no changes are made to the .eml file itself unless I do a Save As to overwrite it or create a new EML file.

That said, I have not yet added any code to programatically create EML files. I have only tested with the sample below, so maybe there is something about the way the file is being generated below that differs from this file, which I got somewhere on the Internet from some Thunderbird example EML file source.

X-Unsent: 1
X-Identity-Key: id1
X-Account-Key: account1
From: "******@papersolve.com" <******@papersolve.com>
Subject: test confirmation
To: ******@papersolve.com
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:69.0) Gecko/20100101
 Thunderbird/69.0
MIME-Version: 1.0
Content-Type: multipart/mixed;
 boundary="------------26A45336F6C6196BD8BBA2A2"
Content-Language: en-US
This is a multi-part message in MIME format.
--------------26A45336F6C6196BD8BBA2A2
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Transfer-Encoding: 7bit
testtest
testtest
testtest
testtest
testtest
testtest
--------------26A45336F6C6196BD8BBA2A2
Content-Type: text/plain; charset=UTF-8;
 name="sha1hash.txt"
Content-Transfer-Encoding: base64
Content-Disposition: attachment;
 filename="sha1hash.txt"
NzRjOGYwOWRmYTMwZWFjY2ZiMzkyYjEzMjMxNGZjNmI5NzhmMzI1YSAqZmxleC1jb25maXJt
LW1haWwuMS4xMC4wLnhwaQpjY2VlNGI0YWE0N2Y1MTNhYmNlMzQyY2UxZTJlYzJmZDk2MDBl
MzFiICpmbGV4LWNvbmZpcm0tbWFpbC4xLjExLjAueHBpCjA3MWU5ZTM3OGFkMDE3OWJmYWRi
MWJkYzY1MGE0OTQ1NGQyMDRhODMgKmZsZXgtY29uZmlybS1tYWlsLjEuMTIuMC54cGkKOWQ3
YWExNTM0MThlYThmYmM4YmU3YmE2ZjU0Y2U4YTFjYjdlZTQ2OCAqZmxleC1jb25maXJtLW1h
aWwuMS45LjkueHBpCjgxNjg1NjNjYjI3NmVhNGY5YTJiNjMwYjlhMjA3ZDkwZmIxMTg1NmUg
KmZsZXgtY29uZmlybS1tYWlsLnhwaQo=
--------------26A45336F6C6196BD8BBA2A2
Content-Type: application/json;
 name="manifest.json"
Content-Transfer-Encoding: base64
Content-Disposition: attachment;
 filename="manifest.json"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--------------26A45336F6C6196BD8BBA2A2--