XG in MTA mode and 3 WAN-Interfaces ... unable to select the correct outbound interface

Which Interface does the XG use? 

DSL1

It uses the IP and the Interface or only the IP and the correct Interface? 

Because you need NAT and SD-WAN PBR. 

Maybe you need a Firewall Rule. Check if there is a MTA Rule. 

It use the incorrect interface.

i have the firewall-rules (and they matches while sending Mails)
my configuration for SD-WAN PBR ist shown in the screenshot.
i have a default-masq rule for all outgouing traffic/interfaces...

Hi dirkkotte :  Please share some packets or information or snapshot where you are able to see incorrect Interface/IP for SMTP/SMTPS traffic.

What is the status of SD WAN policy route for system generated traffic? 

console> show routing sd-wan-policy-route system-generate-traffic

i see the false IP within mail-header of testmails.

console> show routing sd-wan-policy-route system-generate-traffic               
SD-WAN policy route is turned off for system-generated traffic. 

should i enable this?

set routing sd-wan-policy-route system-generate-traffic enable

Hi dirkkotte Before enabling that option I would suggest you may refer some information with reference to that command provided in below KBA :

support.sophos.com/.../KB-000039331

Note: You may test it by enabling that option in non working hours for safer side to avoid any issue/interruption and share the result with us.

Hello,

I tried to enable the setting and it seems that the outgoing emails are being sent correctly (but I'm not sure).

Now, however, the incoming e-mails are no longer transmitted to the internal mail server. (is located within a directly connected network) Is it possible that the SD-WAN-PR also takes over these connections?

Possible because the route-precedence?

console> system route_precedence show
Standard routing priority:
1. SD-WAN policy routes
2. VPN routes
3. Static routes

So I undo the change.

Change the Route precedence to static, vpn, sd-wan. 

This will resolve your internal issue. 

thanks, i'll try this asap.