DNAT Rule for SMTP Traffic

Evening All,

Slight bit of a noob with the Sophos UTM so please bare with me, as i could be doing something completely backwards.

So I have a Sophos SG550 UTM 9 Device, and i'm wanting to pass traffic through from Symantec messagelabs to my Internal Exchange servers, We do not have the Email Protection Licence as the company is to strapped for cash for this. 

Currently have the below interfaces

External Interface within a DMZ area - this is also used to allow Internet traffic out on one IP, have an additional IP for the Exchange traffic to be passed too

Internal Interface - for all internal traffic to access

I currently have a DNAT rule which passes traffic from my external adaptor through to the Exchange servers

Source - Messagelabs IPs

Service - Email Messaging 

Destination - External Exchange 

convert to - Internal Exchange Server

Firewall - Auto Create

Now i can see traffic going through the firewall but symantec moans that it doesn't have a route through i'm assuming this is a problem that it can't get back to symantec, tried adding firewall rules to allow the traffic and setting nat rules but to no help.

So thought i'd throw my questions here to see if anyone could help point me in a decent direction. I'm no network engineer.