Sophos UTM Email Protection Setup Network Design

Hi All,

             I am new to Sophos UTM Email Protection, need some assistance from the experts. I have my Sophos UTM (multiple subscriptions e.g. WAF, Email Protection etc.) in DMZLAN, the (Internal) Email Servers are also in DMZLAN. All network segments are using  another Firewall as Gateway. 

To ensure that both incoming and outgoing Emails are secured/processed by Sophos UTM Email Protection, what I have to do on the Firewall (Gateway) or on Sophos UTM (apart from configuring the Email Protection section described in "Basic Exchange setup with SMTP Proxy"  community blog).

so my main question is how/what traffic  to redirect to Sophos UTM as it is not the Default GW for incoming/outgoing Email traffic. Can someone pleases elaborate detailed steps?

Thanks in Advance