1 x WAN, 1 x VPN Router and SMTP Proxy in MTA Mode Routing Problem

Hi there,
my Problem:

I need to send my SMTP Traffic over an VPN Gateway with is connected to the same WAN as the XG.

Sadly I can't figure out to split up the Traffic into "Everything except SMTP" and "SMTP only". I configured the VPN Gateway as a second Gateway for the UTM.

I need to Route SMTP Traffic from the "Sophos XG SMTP Proxy" in MTA mode through Gateway 2.

Everything else has to take Gateway 1. I saw that the emails are coming from the WAN Interface 192.168.178.2 and are going to Gateway 1.

I tried several NAT - Rules, SD-WAN Policies, Gateway Weights, DCSP markings and several combinations, but sadly it seems like sometimes it works and sometimes it likes to use Gateway 2 for HTTP Traffic, or better VOIP Traffic.

I am coming from an UTM 9.7, where the Multipath Rules did perfectly fine for me. Right now I worked several evenings on this problem, but sadly I can't figure out how to fix this properly.

WAN Setup - "Gateway 1"

1 x DSL Router with IP 192.168.178.1

VPN Router - "Gateway 2 - VPN"

1 x WAN Interface 192.168.178.3 / GW 192.168.178.1

1 x "Internal Gateway" 10.10.10.254

Sophos XG Setup

1 x WAN Interface 192.168.178.2 / GW 192.168.178.1

1 x Internal Interface 10.11.10.254

1 x "VPN" Provider WAN Interface 10.11.12.1 / GW 10.11.12.254

Interal:

1 x Email Server in the 10.11.10.0/24 Network

Best regards,

WepperCapper