Under attack by spammers - need more than only limits returned in Exim Mail Queue on forwarder only messages

I'm working with two servers which have been neglected for years, and have 1000s of email accounts of 100s of email domains. On a daily basis many of the accounts are being spammed by the same group of zombies, so I'm tackling these one by one. I submit the SPAM messages to Spamcop, to get these IPs / people out of the system. Under normal circumstances this is fairy trivial, because I open Webmail, go to the spambox, take the message, view the source, and submit to Spamcop. However, it appears these are some "forwarder only" emails in the system, ie. no mailbox exists for these forwarders. I then try the Download in .eml format option the queue management but get this: "Due to administrative limits only headers are returned" I can't submit just headers, I need to submit the entire message to Spamcop. Does anyone know where these administrative limits are defined?