Anett wrote:

The NDR is created by our back-end server and sent out.

Are you sure your server is generating it or is it just passing it back to
the original sender from an upstream router?

Are the sender and recipient addresses on separate machines? If the router
producing the error is in your domain, then that machine is talking to
another router that it thinks is upstream from it and that other router
thinks the original is upstream so they hand the message back and forth
until the maximum hop value is exceeded causing one of them to report the
NDR to the sender. Is the original message appended to or included in the
NDR? If so, the Received-by headers should tell you which teo machines are
arguing.
--
Brian Tillman