Brian Tillman wrote:
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

According to the information in Message Tracking Center (Exchange
System Manager), the message is transferred from the front end server
to the back end server, and then the NDR is generated.

It's an external sender on a different domain. The original message is
not included in or appended to the NDR.

The thing is the problem occurs only when the mail is addressed to a
send list, not when the sender is addressing it directly to the
recipient. That's why we can't figure out what's wrong. Why it is ok in
one case but not in the other...

Any idea?

Anett