I realise it's probably because of the use of the reject action, which presumably inserts the text "No spamming allowed here." into the subject of the bounce.
However what also concerns me is that sending MDN's back to the envelope sender of SPAM messages is very likely to cause your server to be the origin of "Joe-Job" spam. The sieve RFCs state that if using reject (therefore >sending MDNs) you should also be able to check for forged envelope senders and in that case do not send bounces. In real life it's considered best to just not bother sending anything back at all, and either discard, or block at >the SMTP level using RBLs.
Cheers
Alex
Whatever's the case the backscatter you're talking about has its own ways and methods to be fought with. There are countries for example Germany where it is prohibited by law to discard any email messages silently. You must reject them so that the senders would be aware what is going on. I was told that by one German admin. I am not sure if this is really true but it has some logic on one hand and it is completely groundless on the other hand because you cannot substitute a live person by a machine. Let's say if the machine (computer / server) confirmed reception of the email it does not necessarily mean that any person on the other end also received and read that email. But this is just demagogy that has very vogue legal / law aspects involved.