GWIA "Access Denied" Exchange Coexistence with GW 5.5

[rspettis]

Need some guidance on an integration challenge we've run up against.

We have coexistence successfully running between Exchange 2003 and GW 5.5 via the Gateway API NLM.

Messages are good between the GW POA and the Exchange PO in both directions.

Inbound/Outbound SMTP to/from GWIA is good. Also, inbound/outbound SMTP to/from Exchange SMTP connector is good as well.

But, our GWIA IMAP/POP3 clients cannot send to the Exchange PO. They receive an NDR "Access Denied" for the user they've sent to that resides on Exchange.

GWIA IMAP/POP3 clients can receive inbound message from Exchange PO, but cannot reply.

On a whim, I try sending from GWIA IMAP client to bogus Exchange user, and it indeed NDR's with "User not found"....so it knows the users do/don't exist, just that GWIA thinks they don't have access.

Any guidance from anyone on how to resolve this one?