How to add custom MIME header in email in Exchange online

Mohd Ajaz Khan 0

Hello, We want to know if we can add a custom MIME header in email in Exchange online messages so if we receive the email in Bcc we can redirect that email to a specific mailbox. It is to use SAP application with EXO. SAP has suggested the following article but we can not update the configuration file as we can do in exchange servers. Any suggestion would be valuable.

https://community.sap.com/t5/crm-and-cx-blogs-by-sap/how-to-set-up-a-custom-mime-header-for-your-email-server/ba-p/13579042

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Xintao Qiao-MSFT 6,030 Reputation points Microsoft Vendor

2025-01-17T06:40:34.1366667+00:00

Hi, @Mohd Ajaz Khan

As mentioned in the article you provided, in Exchange Online this may involve creating mail flow rules. To do this, you need to go to the Exchange Admin Center as an administrator, navigate to Mail Flow, and modify your mail flow rules as shown below.

Unfortunately, in Exchange Online, mail flow rules can't detect Bcc recipients directly. This is because the Bcc recipient's information is hidden during the mail transmission process and cannot be accessed by the mail flow rules.

Maybe that's what you're looking for. Configure S/MIME in Exchange Online | Microsoft Learn

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Mohd Ajaz Khan 0 Reputation points

2025-01-17T08:54:23.4+00:00

I have explored this option however our requirement is that we should set this custom header value when senders send emails to a specific recipient mailbox by putting it in Bcc only (not in To or Cc). Any thought on this would be greatly appreciated.

Xintao Qiao-MSFT 6,030 Reputation points Microsoft Vendor

2025-01-20T09:16:31.23+00:00

Hi, @Mohd Ajaz Khan

You mean when someone is only in the BCC (not in the To and CC), add a custom header value to the message and redirect the message to a different mailbox, right?

If that's the case, mail flow doesn't work for that. As mentioned above, because the BCC recipient's information is hidden during the message transmission, the mail flow rules cannot detect the information in the BCC. The BCC detection is not provided in the figure below.

If you need to detect that a specific user is included in the BCC, you may need to use scripts or other tools.

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Mohd Ajaz Khan 0 Reputation points

2025-01-20T10:49:42.0866667+00:00

Thank you. That’s correct, which is why I asked whether the solution mentioned in the SAP article is supported in EXO. Based on your response, it seems that EXO doesn’t offer the options for this, as we don’t have a config file to update like we do with on-premises Exchange servers.

I hope my understanding of your comment is accurate.

Xintao Qiao-MSFT 6,030 Reputation points Microsoft Vendor

2025-01-21T08:47:02.4733333+00:00

Hi, @Mohd Ajaz Khan

Yes, that's right.

To be precise, the scenario provided in the SAP article to add custom header values is feasible. However, Exchange Online does not support the condition that triggers the mail flow (as shown in the figure below), which is the special requirement of detecting the information of the BCC.

If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".

Xintao Qiao-MSFT 6,030 Reputation points Microsoft Vendor

2025-01-24T08:41:56.1866667+00:00

Hi, @Mohd Ajaz Khan

Following up to see if the above answer was helpful. If this answers your query, do click Accept Answer and Yes for was this answer helpful. And, if you have any further query do let us know.
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