You have modified the Bounces Notifications Text in Administration > Configuration > edit Scanner > SMTP > Advanced Settings button > Inbound : Bounces Notifications Text
You have also created a "Send a Delivery Status Notification with explanation" action in a custom content filter.
You have noticed that the Delivery Status Notification (DSN) generated by your custom content filter action contains default text and does not contain your modified Bounces Notifications Text.
When setting Content Filtering Policy whose Action is a 'Send a Delivery Status Notification', message sent by Mailer-Daemon@hostname will include the value input manually by user in the 'Explanation' field, as well as a default DSN message similar to Bounce Notifications Text (in SMTP Advanced Settings) as shown below.
"I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.
For further assistance, please send mail to $postmaster$
If you do so, please include this problem report. You can
delete your own text from the attached returned message."
Release : 10.8.0
This is currently working as designed. The mechanisms for the Bounce Notification DSN text generated directly by the mail server differ from those used when generating the DSN by content filters. The DSN generated by content filters allows for explanation text to be added to the default DSN text.
Utilizing the Bounce Notification Text in the content filter DSN will be available in a future release. This article will be updated as new information is available.