why our server got listed?

Today our primary mail server got listed again (it was delisted
yesterday).  Server's ip is 195.54.192.35
Reason of listing is:
System has sent mail to SpamCop spam traps in the past week

Dispute listing didnt work - noone care to answer.

How can i get any info about reasons of listing? This system does
not originate mail itself, it's just mail relay.

P.S. while reading spamcop web site i've found 'misdirected bounce'
feature. Can anyone explain me how it can be avoided on secondary
mail relays (which do not have any info about quotas/existing users 
etc. and _can not_ reject mail during smtp phase)?

-- 
Oleg.