I am having problems fighting NDR (backscatter) spam on Exchange 2007 server.
I have been using Exchange Antispam modules for few years now. But last week client started to receive hundreds of NDR (apparently created by my Exchange 2007 server). It is unlikely that is he sending spam.
1. I have tested my SPF records. It all looks OK.
2. I ve tested that it is not a open relay.
3. I have disabled "Allow non delivery reports" on Hub Transport default remote domain.
4. Tried to restart services on Hub transport role server.
NDR spam still arrives as like nothing has changed. Is there anything else i can do?