Suppose the original message, copied into the bounce,
contains pornography which is illegal under English law.
First off, there is nothing that requires you to copy anything into the
bounce.
Secondly if it is an automatic process your liability would be no greater
than it would be following original SMTP.
There is no English law against 'pornography', the actual offense is
publication of obscenity.
There is a whole rack of issues here, first publication on the Internet has
been addressed in the English courts, there has to be actual knowledge or at
the very least a constructive effort to avoid knowledge of the contents for
publication to have occured. Then when you get into the obscenity
definitions you have a whole rack of further intent issues.
If you are worried about creating liability don't copy information.