strange 550 error

[Chris Yuzik]

Suddenly today, we're getting users complaining that when they do a 
send/receive in outlook, they get a message that says:

"Sending reported error (0x800CCC69) The server responded 550 5.7.1 DSN 
or MDN for message that did not originate here"

We haven't made any changes to the server for a couple of weeks. I'm not 
sure whether this would be an issue with MailScanner's watermarking 
feature or whether it would be Milter-null. In any case, does anyone 
have any ideas how to stop these strange messages?

Thanks.