How do you resolve message wrapping that shows as attachment when transport rule is enabled for external email warning?

Question

Scenario:

1. An email is created in Gmail that utilizes SMIME encryption certificates.
2. Email is sent to an outlook user that has transport rule enabled for message to warn of external message receipt.
3. Exchange executes rule to prepend the warning message to the sender email. This action appears to break the digital certification (the message is altered) In lieu of breaking the digital certification, the original message is changed to an attachment. The warning shows in the inbox message body and the actual email originally sent is an attachment.
4. This is constrains user time and makes email searching pointless.

Is there a fix for this issue that I have seen many conversations on but no solutioning?

Answer 1

Hi @Dawn Rose ,

Exchange executes rule to prepend the warning message to the sender email. This action appears to break the digital certification (the message is altered) In lieu of breaking the digital certification, the original message is changed to an attachment. The warning shows in the inbox message body and the actual email originally sent is an attachment.

As you may have noticed in some other conversations about the similar topics, this is actually a normal behavior.

Digital certificates are intended to ensure the integrity of the message and it proves that the message body and attachments are not modified between the point of sending and arriving. While the disclaimer transport rule you used works by altering the message body, so it's expected that the digital certification is no longer valid, and the email is finally wrapped as an attachment. This is all by design.

With the above being said, I'd like to recommend having a go by utilizing the External Email Warning Tag feature and see if it can be an alternative for your situation.

Set-ExternalInOutlook –Enabled $true

From what I know, the new Outlook native callouts works by adding a new MAPI property called IsExternalSender to the email item, so seems like it won't affect the message body.
More details:

• Native external sender callouts on email in Outlook
• Enable External Email Warning Tag in Exchange Online
  (Please Note: Since the web site is not hosted by Microsoft, the link may change without notice. Microsoft does not guarantee the accuracy of this information.)

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