External User receiving many NDRs

Hi,

I have a problem with NDRs on Exchange 2013. A user received an external email and the sender keeps receiving many NDRs. Since last Wednesday he got about 650 NDRs of that message.

The strange thing about this, is that my user received  the message, but the sender (external one) keeps receiving NDRs of "message exceeded the limit"

My problem here is not related to the message, but about this repeating of the NDR to the sender.

Please Help.