Mail Relay for one customer on a different IP as for the rest of the SMTP traffic

Question

Status: 
 
 Sophos UTM cluster with two WAN interfaces/IP-networks 
 Mail protection in use for all email traffic 
 Outgoing SMTP traffic on the primary address of the 1st WAN interface 
 Customer's MX record points to above IP address and is processed by Exchange server in LAN 
 
 Requirement: 
 
 Customer wants to send great amount of automatically generated report emails 
 To avoid emails going through the Exchange server, the customer's program will connect to Sophos UTM SMTP Proxy from WAN with Username/Password. 
 We want to avoid to have our primary SMTP IP blacklisted if customer's emails cause trouble 
 
 Question: 
 
 Is there a way to have only this email stream leave the UTM from a different IP address/interface than our 1st WAN's primary IP?

DouglasFoster · Answer

You need to bypass the SMTP Proxy so that the traffic flows out with DNAT. If your source server does not do mail routing, and you don't want to use an Exchange send connector, you can download a copy of SmarterMail from SmarterTools.com, and use it as a mail relay.