That is NDR spam - pure and simple.
A good Anti-Spam program will be able to eliminate these for you.
Have you got anything installed on your server?
If not, please download a trial of Vamsoft ORF - www.vamsoft.com - Excellent anti-sapm software and should deal with this issue for you.
If you decide to keep it, it will only cost you $239 as a one-off cost.
If you need help configuring it, please ask. I use it on all my customer sites and it is brilliant.