NAT and multihoming

By francesco.ruggeri ·
Hi all, I have a question I'm thinking about since some period with no results and great headache.
I have a multihomed Cisco router: it is connected to two different ISPs. Each ISP has assigned ONE public IP address, say A and B.
The router ethernet interface connects directly to my private lan where I have clients and servers running internal services like Exchange, IIS, FTP.... and others. Say X internal IP address.
I want to make internal services being accessed from outside using both ISP's addresses, A and B.
No problem configuring static nat for my services from A to X and from B to X (I've used an "ip nat inside source static tcp X 25 A 25 extendable", the same for B.)
The issue is the following: response packets sourced from X are not routed back throught the same interface they came in (correct behavior, I think...) but accordingly to routing table (in my routing table both ISPs are installed with the same metric). Nothing changes if i try with policy routing. My feelings is that this is not a matter of routing... NAT instead?
Any help appreciated.

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