publish exchange server on second wan interface with static ip address

Hello experts,

following configuration:

UTM with 1 LAN-Interface and 2 WAN-Interfaces (1 x ADSL 50 MBit/s = default GW (Interface A), 1 x ADSL 16 MBit/s with static IP (Interface B)).

I want to publish the Exchangeserver over the second WAN-Interface with static IP (Interface B), but proxy all other traffic over the first WAN-Interface (Interface A). When setting up the default gateway on the interface A, the published Exchangeserver services like OWA and ActiveSync are not reachable. Changing the default gateway to Bnterface B the services are reachable.

How can I configure the UTM to publish the Exchangeserver over the Interface B but all other (outgoing) traffic over Interface A?

Regards

Stephan