When the bounced message is set in Plesk, the actual bounced message looks as follows:
This is the mail system at host example.com
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.
Delivery has failed to these recipients or groups:
There's a problem with the recipient's mailbox. Please try resending this message. If the problem continues, please contact your helpdesk.
Diagnostic information for administrators:
Generating server: example.com
Is it possible to modify the full text of bounce text message?
Such messages are generated by the mail transfer agent on the side which receives the bounced message.
The mail server on the Plesk side cannot influence this behavior.