BizTalk HL7 Request Response Acknowledgement Problem

The scenario:

I’m using BizTalk 2006 R2 to receive an HL7 request and return an HL7 response.   I’m using a two-way receive port MLLP etc and the orchestration ports are bound to this port.  All of this works fine, and I can successfully fire an HL7 request and return an HL7 response.

The problem:

The specification, quite rightly, demands that errors should return a NACK.  As I understand this the good response has MSA.1 as AA and this response is also the positive acknowledgement (ACK) so we don’t want BizTalk to generate its own ACK just return the response message as the ACK, but generate and return a NACK on error.

I’ve  used BTAHL7 Configuration Explorer Acknowledgement tab to try and configure this but without success.  Is it possible to configure BizTalk to just return the NACKs if an error occurs otherwise returning the good message on this two-way port?

Any help/advise much appreciated.