Exchange (online) email header "X-MS-Exchange-Transport-Forked: True"

zerasar 1 Reputation point
2021-03-22T22:10:35.48+00:00

Hiya!

At my workplace we mandate that emails are encrypted with SMIME certificates. This is only for internal emails as we are aware many external recipients often have issues opening signed or encrypted emails.

In passing my manager suggested that I put a mailflow/transport rule in that would enforce this.

With a lot of trial and error I believe that I got this functioning correctly. Test results below.

Internal to internal unencrypted - Blocked
Internal to internal encrypted - allowed
Internal to internal & external unencrypted - allowed.

The third scenario was complicated as it became clear that each email was being enumerated against the rules individually rather than collectively. So found it hard to find a condition/exception that allowed me to identify this specific recipient scenario.

Comparing the headers of the different scenarios the third scenario I noted "X-MS-Exchange-Transport-Forked: True". This is what I eventually ended up using as an exception to my mail blocking rule and appears to be working ok.

My issue though, is that I am having a hard time locating documentation on specifically what this header is, how it's used, and what the values represent.

In this instance it appears to mark an email that is sent both internally and to a separate domain, which is what I want... But I want to make sure I fully understand this and am not going to break something down the line.

TLDR
What is this header?
How is it used?
What do the values represent?

Microsoft Exchange Online Management
Microsoft Exchange Online Management
Microsoft Exchange Online: A Microsoft email and calendaring hosted service.Management: The act or process of organizing, handling, directing or controlling something.
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Exchange Server Management
Exchange Server Management
Exchange Server: A family of Microsoft client/server messaging and collaboration software.Management: The act or process of organizing, handling, directing or controlling something.
7,335 questions
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2 answers

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  1. Andy David - MVP 141.1K Reputation points MVP
    2021-03-22T23:06:51.543+00:00

    Forking an email traditionally is the same as bifurcation
    https://learn.microsoft.com/en-us/exchange/mail-flow/mail-routing/recipient-resolution?view=exchserver-2019#bifurcation

    $True, well, seems apparent :) - If the message is forked - split- bifurcated- two copies created - then I expect to see that set to true.

    in this case, the message was forked ( bifurcated) between internal and external users.

  2. Joyce Shen - MSFT 16,641 Reputation points
    2021-03-23T06:20:33.69+00:00

    Hi @zerasar

    According to my search, seems not finding the official document which introduces about the message header X-MS-Exchange-Transport-Forked, and I my previous thread, it's hard to mark the message send to both internal and external recipients. Just like this: Exhange 2010 limiting message size for internal users only

    The rule works fine when a message is sent to internal or external ONLY.

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