shared SMTP namespace, looping NDR

Migrating from LotusNotes to exchange 2010.  Have both servers running and sharing SMTP namespace.  Mail works fine but there is an issue with NDR messages looping between the two mail servers.

Each server is not authoritative and relays emails with unkown addresses at our domain  to each other.
In this way mail can flow between LotusNotes users and Exchange users both with the same domain name.  The problem is with messages to addresses that don't exist on either server.
Since neither server is authoritative for the domain they won't NDR anything and thse emails just loop between the two servers.

How can I fix this?
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hscdsbAuthor Commented:
Everything I have read says to make the last server in the chain authoritative.  While this would work
for incoming external mail, it woudl break internal mail flow between exchange and lotusntoes users.
Disable sending of NDRs on both servers.

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hscdsbAuthor Commented:
How does that help me.  Currently after some time Exchange will see that there is a loop and hold the email but the user never gets a NDR.  I want a NDR to be sent as usual.
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are you looking to run in co-existence or complete the migration soon?

hscdsbAuthor Commented:
The migration will take some time.  We want both servers to run at the same time.
Take a look at this article, in particular it talks about the transporter suite and the directory connector.

Can you confirm all the tools to assist with co-existence are functioning and no errors in event log.

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