dose it possible to use a dedicated line for (GSSMO and SMTP) Upload only?

Question

Hello 
 I'm using Sophos UTM SG210 with Two ADSL Lines (Slow Uploads) , that make sending emails take very long time, 
 and i plan to add a 3rd 4G line (very fast Upload, but Limit Quota), 
 
 the two ADSL Line interface work in Load balancing, 
 
 are it possible to use this 4G line for (GSSMO and SMTP) upload only, i don't want any thing else to use from this line Quota, 
 
 and keep the other lines work, and backup this 4G line if fail. 
 
 if it possible please help with the right way to setup this, 
 
 Thanks in advance. 
 Ahmed Fawzi

LuCar Toni · Answer

Ok lets start from the beginning. 
 
 Multipath is only available for Uplink Interfaces. So basically you put all your "WAN" Interfaces in the Uplink balancing.

if you do not configure any Multipath rule, UTM will load balance everything depending on the little wrench.

You can influence this with Multipath, because multipath rules will "overwrite" the load balancing. 
 
 So you will most likely put your unwanted Interface on 0 in the scheduler and create the Multipath Rule like explained.

This will bind the Traffic SMTP no matter what to the Interface. 
 
 Now every SMTP connection outgoing should use the Interface selected above and only this interface. You should not see any traffic except SMTP on this interface.