I've seen this before when an Exchange SMTP Server setting was set up to allow external relaying.
In our case we enabled the authenticated SMTP (EHLO) processing settings in our SMTP/400 product and were able to relay after authenticating.
You might need to ask the network folks what may have changed.
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date: Thu, 14 Apr 2011 11:10:55 -0500
from: "Don Cavaiani" <dcavaiani@xxxxxxxxxxxxx>
subject: SMTP non-delivery report
I have a CLP which does a SNDDST (then hands to Exchange Server) - to
occasionally send a fixed message to about 400 Suppliers.
I have run this many times in the past, but now it only seems to be working if I use an Email address of one of our internal Users.
If I now use an external email address, the exchange server message tracking log reports: SMTP: 'Non-Delivered Report (NDR) generated', and no email goes to the external address.
However, email mail routing exclusively thru exchange IS working both ways - inbound and outbound.