A DSN (Delivery Status Notification ) is a special formatted message and is defined in RFC 1891.
The basic idea behind a DSN is to create a message that is readable by a human and by a machine.
The general structure of the message can not be changed, but you can change the text to some extend.
Download the default text file from here and open smtpdsn.txt in Notepad.
You will see a lot of sections and each section consist of a error number ( for example 5.1.1 ), a one line short description that is used in the subject of the DSN and a multi line text that is added to the body of the DSN.
Now change or add whatever you need and once you are finished copy smtpdsn.txt into the directory where MBServer.exe resides.
Also enclosed you will find smtpdsn-english-spanish.txt, smtpdsn-english-french.txt, smtpdsn-english-german.txt and smtpdsn-english-italian.txt which are translations of smtpdsn.txt. If you want to use it then rename it to smtpdsn.txt and copy it into the directory where MBServer.exe resides.
Note: if you make your own translation then always keep the English text as is, because not everyone may be able to read and understand your language.