Static NAT and mail server on LAN

Hi everybody

NW 6.5 SP8 box with BM 3.9 SP2 and Netmail 3.1 configured to service
mail for mydomain.com on the other box on LAN.
In BM box there is public NIC with primary IP address: XXX.YYY.ZZZ.ABC
and secondary IP address: XXX.YYY.ZZZ.ABD. "Static and dynamic NAT"
enabled on primary address, static NAT set up between XXX.YYY.ZZZ.ABD
and 192.168.0.4 where my Netmail mail server is listening. In DNS an A,
PTR and MX records for secondary IP address are set up. For primary IP
address only an A and PTR records are set up.
Static NAT is working, I can connect from internet to my internal mail
server without any problems.
The problem is when my mail server sends message to the internet, some
servers say: host not allowed to send mail for mydomain.com. I realized
that Netmail sends email via dynamic NAT and primary public address but
MX record is only configured for secondary IP address.
Default route for Netmail is private IP address of BM box.

Where is the problem?
Should I also set MX record for primary public IP address
XXX.YYY.ZZZ.ABC? (but mail server do not listen on this interface)
Or how can I enforce Netmail to send email by secondary IP address on
public NIC of BM box? Is it possible?
I know that I can set up mail proxy or generic proxy for SMTP on primary
public IP but I prefer to use static NAT (I think it is more stable).

Regards

Pawel