This is a quick one because i know you have no time to look into how mail servers work in extreme detail, so here it is.
This is a fast resolution to your backscatter problem, yet, after this resolution, it might take the spammer some time to realize that your server is no longer including the original message in the responce.
postconf -e 'bounce_size_limit = 1'
What this does is that it makes your message bounce with the headers but not the body of the message, which makes the spammer who is targeting your server loose interest *(because she can not make use of the bounced message if the body is missing)
right after you need to
/etc/init.d/postfix reload /etc/init.d/postfix restart
Your choice 🙂
And this is the fastest solution, this is because any other solution will require you to investigate the whole setup, while this solution is a simple solution that is still effective.