backscatter by DSN: Service unavailable

[hofu]

Hi all,

our primary mailserver tags the spam and relays mail to other mailservers.
Those sometimes have a different view of accepting messages
and frequently reject spam mail 
(different view of DNS, different RFC inforcement). The primary server 
backscatters the tagged spam to falsified sender addresses.
I am already rejecting spam from the subdomains mailservers with mailscanner
upion reaxamining the mail
but this happens as 
DSN: Service unavailable
which I can't seem to intercept with mailscanner.

Any chance to do this via watermarks or some such?
Thanks 
hofu