Lol... Chicken, meet egg. I think they're honestly trying to cut back on malware-generated spam from the source in order to keep off the blacklists.
How are you including the spam? As an attached email (.eml) or text file? Are you just copy/pasting the headers into your email? The reason why I ask is there may be a sensible way to get around the detection by compressing it before attaching it - though that might also preclude some abuse departments from accepting it.
To be honest, I stopped reporting spam since some cloaked spamhauses (and malware-infected hosting sites) started collecting the emails to send more spam. What I do is report the IP it was last sent from (which is pretty hard to spoof) to something like AbuseIPDB and/or a blacklist like spamhaus.org.