Unable to Process Notification Responses From Outlook Email. Responses Move to DISCARD Folder (Doc ID 341677.1)

Symptoms

Notification Mailer is not able to process all responses.
Majority valid of responses are being sent to DISCARD.
If e-mail client tool generates the response in Plain Text the response is processed successfully.
If e-mail client tool generates the response in HTML the response is valid and still moved to DISCARD folder. In this case responder receives a response back indicating ... "Warning for Unsolicited Email".

Problem happened right after upgrading to 11.5.10 (CU2) from 11.5.9.

Steps to Reproduce:

1. Send Test Notification from Workflow Mailer.
2. Open Notification in MS Outlook.
3. Click on "Acknowledge" from the notification email.
4. Send the resulting response email.
5. Get error email with subject "Warning For Unsolicited Mail".
6. Verified in the IMAP mailbox that the response email went into the DISCARD
folder instead of getting PROCESSED.

Changes

MS Outlook is the email client
MS Outlook is set to respond usig HTML editor
This could happen using other e-mail client tools that respond in HTML format.
Upgrade the instance

Cause

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